Biopolymers and Biopolymer Blends: Fundamentals, Processes, and Emerging Applications 9781032542607

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Biopolymers and Biopolymer Blends Biopolymer and Biopolymer Blends: Fundamentals, Processes, and Emerging Applications showcases the potential of biopolymers as alternative sources to conventional nonbiodegradable petroleum-based polymers. It discusses fundamentals of biopolymers and biopolymer blends from natural and synthetic sources, synthesis, and characterization. It also describes development of desired performance for specifc applications in 3D printing and other emerging applications in industry, including packaging, pulp and paper, agriculture, biomedical, and marine. • Introduces the fundamentals, synthesis, processing, and structural and functional properties of biopolymers and biopolymer blends. • Explains the fundamental framework of biopolymer blends in 3D printing, featuring current technologies, printing materials, and commercialization of biopolymers in 3D printing. • Reviews emerging applications, including active food packaging, electronic, antimicrobial, environmental, and more. • Discusses current challenges and futures prospects. Providing readers with a detailed overview of the latest advances in the feld and a wealth of applications, this work will appeal to researchers in materials science and engineering, biotechnology, and related disciplines.

Emerging Materials and Technologies Series Editor: Boris I. Kharissov The Emerging Materials and Technologies series is devoted to highlighting publications centered on emerging advanced materials and novel technologies. Attention is paid to those newly discovered or applied materials with potential to solve pressing societal problems and improve quality of life, corresponding to environmental protection, medicine, communications, energy, transportation, advanced manufacturing, and related areas. The series takes into account that, under present strong demands for energy, material, and cost savings, as well as heavy contamination problems and worldwide pandemic conditions, the area of emerging materials and related scalable technologies is a highly interdisciplinary feld, with the need for researchers, professionals, and academics across the spectrum of engineering and technological disciplines. The main objective of this book series is to attract more attention to these materials and technologies and invite conversation among the international R&D community. Wastewater Treatment with the Fenton Process Principles and Applications Dominika Bury, Piotr Marcinowski, Jan Bogacki, Michal Jakubczak, and Agnieszka Jastrzebska Mechanical Behavior of Advanced Materials: Modeling and Simulation Edited by Jia Li and Qihong Fang Shape Memory Polymer Composites Characterization and Modeling Nilesh Tiwari and Kanif M. Markad Impedance Spectroscopy and its Application in Biological Detection Edited by Geeta Bhatt, Manoj Bhatt and Shantanu Bhattacharya Nanofllers for Sustainable Applications Edited by N.M Nurazzi, E. Bayraktar, M.N.F. Norrrahim, H.A. Aisyah, N. Abdullah, and M.R.M. Asyraf Chemistry of Dehydrogenation Reactions and its Applications Edited by Syed Shahabuddin, Rama Gaur and Nandini Mukherjee Biopolymers and Biopolymer Blends Fundamentals, Processes, and Emerging Applications Abdul Khalil H. P. S., Nurul Fazita M. R., and Mohd Nurazzi N. For more information about this series, please visit: www.routledge.com/EmergingMaterials-and-Technologies/book-series/CRCEMT

Biopolymers and Biopolymer Blends Fundamentals, Processes, and Emerging Applications

Abdul Khalil H. P. S., Nurul Fazita M. R., and Mohd Nurazzi N.

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

First edition published 2024 by CRC Press 2385 Executive Center Drive, Suite 320, Boca Raton, FL 33431 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2024 Abdul Khalil H. P. S., Nurul Fazita M. R., and Mohd Nurazzi N. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microflming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www. copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identifcation and explanation without intent to infringe. ISBN: 978-1-032-54260-7 (hbk) ISBN: 978-1-032-54265-2 (pbk) ISBN: 978-1-003-41604-3 (ebk) DOI: 10.1201/9781003416043 Typeset in Times by Apex CoVantage, LLC

Contents Preface.................................................................................................................... xiii Authors..................................................................................................................... xv Chapter 1

Biopolymer Composites: Processing, Surface Modifcation and Characteristics ...............................................................................1 1.1 1.2 1.3 1.4

1.5

1.6 1.7

1.8

Introduction ...............................................................................1 Retrospective on Biopolymers...................................................3 Defnition of Biopolymers .........................................................5 1.3.1 Research Trends for Biopolymers ................................ 7 1.3.2 Market Trends for Biopolymer-Based Products........... 8 Biopolymer Blends and Bionanocomposites ............................. 8 1.4.1 Research Trends for Biopolymer Blends and Bionanocomposites..................................................... 10 1.4.2 Pros and Cons of Biopolymer Blends and Bionanocomposites..................................................... 14 Natural Biopolymers ............................................................... 15 1.5.1 Plant/Algae-Based Polysaccharides .......................... 17 1.5.2 Animal-Based Polysaccharides .................................20 1.5.3 Microorganism-Based Polysaccharides .................... 22 Chemical Synthesis ................................................................. 27 1.6.1 Renewable Resources ................................................. 27 1.6.2 Petroleum-Based Resources ....................................... 30 Natural Polysaccharides .......................................................... 31 1.7.1 Cellulose..................................................................... 31 1.7.2 Starch.......................................................................... 33 1.7.3 Pectin..........................................................................34 1.7.4 Agar............................................................................ 36 1.7.5 Carrageenan ............................................................... 37 1.7.6 Alginate ...................................................................... 38 1.7.7 Chitosan...................................................................... 39 1.7.8 Hyaluronic Acid .........................................................40 1.7.9 Yeast Glucan............................................................... 41 1.7.10 Pullulan ...................................................................... 42 1.7.11 Dextran....................................................................... 43 1.7.12 Bacterial Cellulose .....................................................44 1.7.13 Xanthan Gum ............................................................. 45 Natural Proteins.......................................................................46 1.8.1 Soy Protein ................................................................. 47 1.8.2 Zein ............................................................................48 1.8.3 Gluten ......................................................................... 49 1.8.4 Collagen...................................................................... 50 v

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1.9

1.10

1.11 1.12

1.13

1.14 1.15 1.16 1.17 1.18

1.19 Chapter 2

1.8.5 Gelatin ........................................................................ 51 1.8.6 Keratin........................................................................ 52 Synthetic Polymers from Renewable Resources ..................... 53 1.9.1 Polylactic Acid............................................................ 53 1.9.2 Polyhydroxybutyrate................................................... 54 1.9.3 Polybutylene Succinate............................................... 55 1.9.4 Bio-Polyethylene......................................................... 56 Synthetic Polymers from Petroleum-Based Resources ........... 57 1.10.1 Polycaprolactone......................................................... 57 1.10.2 Polyvinyl Alcohol....................................................... 58 1.10.3 Polybutyrate Adipate Terephthalate ........................... 59 Polymer Blends........................................................................ 61 Methods of Polymer Blending.................................................64 1.12.1 Melt Blending .............................................................64 1.12.2 Mill Mixing and Fine Powder Mixing Techniques.................................................................. 65 1.12.3 Solution Casting Method............................................ 65 1.12.4 Freeze Drying.............................................................66 1.12.5 Latex Blending ...........................................................66 1.12.6 Interpenetrating Polymer Networks ........................... 67 Properties of Polymer Blends .................................................. 67 1.13.1 Electrical Properties................................................... 67 1.13.2 Mechanical Properties................................................ 68 1.13.3 Thermal Properties..................................................... 68 1.13.4 Optical Properties....................................................... 69 Blending Biopolymers with Other Biopolymers ..................... 69 Blending of Natural Biopolymers with Other Natural Biopolymers............................................................................. 69 Blending of Synthetic Biopolymers with Other Synthetic Biopolymers............................................................. 74 Blending of Natural Biopolymers with Synthetic Biopolymers............................................................. 76 Surface Modifcation of Blending ........................................... 78 1.18.1 Copolymerization ....................................................... 79 1.18.2 Grafting ......................................................................80 1.18.3 Transesterifcation ......................................................80 1.18.4 Reactive Coupling Agent............................................ 81 Conclusions.............................................................................. 81

Biodegradation and Compostable Biopolymers ............................... 105 2.1 2.2 2.3 2.4

Biodegradation and Compostables in General ...................... 105 Methods of Biodegradation and Composting of Biopolymers ...................................................................... 108 Biodegradability of Biopolymers .......................................... 111 Composting of Biopolymers.................................................. 114

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2.5 2.6 Chapter 3

State-of-the-Art Natural Biopolymers for Bionanocomposites........................................................................... 125 3.1 3.2 3.3

3.4

3.5

3.6 Chapter 4

Environmental Impact of Biopolymers ................................. 115 Conclusions............................................................................ 117

Bionanocomposites................................................................ 125 Constituents of Bionanocomposites ...................................... 127 Types of Bionanocomposites ................................................. 129 3.3.1 Bionanocomposite Films.......................................... 129 3.3.2 Bionanocomposite Hydrogels................................... 136 3.3.3 Bionanocomposite Aerogels..................................... 139 Fabrication Process of Biopolymer-Based Nanocomposites..................................................................... 141 3.4.1 Direct Mixing Approach .......................................... 143 3.4.2 Melt Blending Approach .......................................... 144 3.4.3 In Situ Polymerization.............................................. 145 3.4.4 Sol-Gel Approach..................................................... 146 The Properties of Bionanocomposites................................... 148 3.5.1 Physical Properties ................................................... 148 3.5.2 Mechanical Properties.............................................. 150 3.5.3 Thermal Properties................................................... 151 3.5.4 Biodegradable Properties ......................................... 151 Conclusion ............................................................................. 152

Biopolymers in 3D Printing Technology.......................................... 161 4.1

4.2

4.3

3D Printing Technologies ...................................................... 161 4.1.1 Fused Deposition Modelling .................................... 163 4.1.2 Liquid Deposition Modeling .................................... 165 4.1.3 Stereolithography ..................................................... 165 4.1.4 Laminated Object Manufacturing............................ 166 4.1.5 Composite-Based Additive Manufacturing.............. 166 4.1.6 Powder Bed Fusion................................................... 167 4.1.7 Binder Jetting ........................................................... 167 3D Printing Materials............................................................ 168 4.2.1 Thermoplastic Polymers........................................... 168 4.2.2 Thermosetting Polymers .......................................... 169 4.2.3 Metals ....................................................................... 171 Biopolymers in 3D Printing................................................... 172 4.3.1 Poly (Lactic Acid) .................................................... 172 4.3.2 Poly (Lactic Acid)/Poly (Butyleneadipate-CoTerephthalate) Polymer Blends in 3D Printing......... 173 4.3.3 3D Printing of Native Cellulose-Based Materials................................................................... 174 4.3.4 3D Printing of Cellulose Derivative-Based Materials................................................................... 176

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4.3.5

4.4 Chapter 5

Applications of Biopolymer Blends and Biopolymer-Based Nanocomposites ............................................................................... 193 5.1 5.2 5.3 5.4 5.5

5.6 5.7

5.8 5.9 5.10 5.11 5.12 Chapter 6

3D Printing of Cellulose Composite-Based Materials................................................................... 178 4.3.6 3D Printing of Hemicellulose-Based Materials ....... 180 4.3.7 3D Printing of Starch-Based Materials .................... 180 4.3.8 3D Printing of Algae-Based Materials..................... 183 4.3.9 3D Printing of Chitosan-Based Materials ................ 184 Conclusions............................................................................ 185

Introduction ........................................................................... 193 Building and Construction .................................................... 194 5.2.1 Superplasticizers ...................................................... 194 5.2.2 Insulated Materials................................................... 197 Marine and Coastal Construction..........................................200 Geotextiles.............................................................................200 Agriculture ............................................................................204 5.5.1 Biodegradable Mulches ............................................204 5.5.2 Seed Coatings...........................................................206 5.5.3 Soil Conditioners......................................................208 5.5.4 Organic Fertilizers ...................................................209 5.5.5 Pesticides .................................................................. 210 Packaging .............................................................................. 212 5.6.1 Food Packaging ........................................................ 213 5.6.2 Non-Food Packaging................................................ 215 Cosmetics............................................................................... 217 5.7.1 Skin Care.................................................................. 217 5.7.2 Face Masks/Sheets ................................................... 219 5.7.3 UV Sunscreens ......................................................... 221 Haircare Products.................................................................. 223 Biomedical Applications .......................................................224 5.9.1 Implant and Scaffold Tissue Engineering ................ 225 5.9.2 Wound Healing......................................................... 226 Pharmaceutical Applications................................................. 231 5.10.1 Drug Delivery........................................................... 234 Automotive ............................................................................ 236 Conclusions............................................................................ 239

Starch-Based Films with Essential Oils for Antimicrobial Food Packaging ................................................................................254 6.1 6.2 6.3

Introduction ........................................................................... 254 Characteristics of Starch ....................................................... 255 Essential Oils......................................................................... 258 6.3.1 Extraction Methods .................................................. 259

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Contents

6.4 6.5 6.6 Chapter 7

Chitosan-Based Chemical Sensors: Sensing Mechanism and Detection Capacity ........................................................................... 273 7.1 7.2 7.3 7.4 7.5 7.6 7.7

Chapter 8

Introduction ........................................................................... 273 Chitin and Chitosan: Structure and Characteristics.............. 274 Preparation of Chitosan......................................................... 276 Surface Modifcation of Chitosan.......................................... 276 Role of Chitosan-Based Chemical Sensors ........................... 277 Chitosan-Based Chemical Sensor Sensing Mechanisms ...... 278 Conclusions............................................................................280

Seaweed-Based Biopolymers for Sustainable Applications.............284 8.1 8.2 8.3

8.4

8.5 8.6 8.7 8.8 Chapter 9

6.3.2 Antimicrobial Activity of Essential Oils..................260 Preparation of Starch-Based Films with Essential Oils ........ 261 Performance of Antimicrobial Starch-Based Films with Essential Oils ...................................................... 263 Conclusions............................................................................ 267

Introduction ...........................................................................284 Seaweed Polysaccharides ...................................................... 286 Types of Seaweeds................................................................. 288 8.3.1 Red Seaweed ............................................................ 288 8.3.2 Green Seaweed......................................................... 290 8.3.3 Brown Seaweed ........................................................ 290 Seaweed Derivatives.............................................................. 291 8.4.1 Alginate .................................................................... 291 8.4.2 Carrageenan ............................................................. 292 8.4.3 Agar.......................................................................... 294 Properties of Seaweed-Based Biopolymers........................... 295 Applications of Seaweed ....................................................... 297 Challenges and Future Prospects ..........................................300 Conclusion ............................................................................. 301

Characteristics and Performance of Emerging Biopolymers from Sugar Palm Starch for Packaging............................................308 9.1 9.2 9.3 9.4 9.5 9.6

Introduction ...........................................................................308 Types of Packaging Materials ...............................................309 9.2.1 Production of Biobased Plastics ............................... 310 9.2.2 Biopolymers from Starch ......................................... 311 Sugar Palm Fiber ................................................................... 313 Sugar Palm Starch ................................................................. 313 Preparation of Sugar Palm Starch ......................................... 314 Characteristics of Sugar Palm Starch–Based Biopolymers........................................................................... 315

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Contents

9.7

9.8 9.9

Process of Modifying Sugar Palm Starch and Its Purposes ................................................................................ 316 9.7.1 Mechanism of Plasticization for Starch ................... 317 9.7.2 Plasticization of Sugar Palm Starch ......................... 319 Performance of Sugar Palm Starch Composites ................... 321 Conclusions............................................................................ 326

Chapter 10 Biopolymers for Drug Delivery Applications: Modifcations and Performance .............................................................................. 331 10.1 10.2 10.3

10.4 10.5 10.6 10.7 10.8

Introduction ........................................................................... 331 Classifcation and Characteristics of Biopolymers ...................................................................... 334 Modifcation Techniques for Enhancing Biopolymer Properties............................................................................... 336 10.3.1 Glutaraldehyde ......................................................... 339 10.3.2 Poly(Carboxylic Acids)............................................. 339 Crosslinking of Biopolymers with Polycarboxylic Acid–Based Crosslinkers ......................................................340 Fabrication Process of Biopolymers...................................... 343 Biopolymers in Drug Delivery Applications......................... 345 Release Characteristics of Biopolymers ................................ 349 Conclusion ............................................................................. 350

Chapter 11 Crosslinking Networks of Functional Biopolymer Hydrogels.......................................................................................... 357 11.1 11.2 11.3 11.4 11.5

Introduction ........................................................................... 357 Overview of Biopolymer Hydrogels......................................360 Importance of Surface Functionalization..............................360 Crosslinking Networks for Functionality of Biopolymer Hydrogels............................................................................... 363 Conclusions and Future Perspectives .................................... 363

Chapter 12 Current Challenges and Future Prospects of Biopolymer Blends and Biopolymer-Based Nanocomposites.............................. 366 12.1 Introduction ........................................................................... 366 12.2 Biopolymer Derivatives and Processing Methods ................................................................................. 367 12.3 Production at Mass Scale ...................................................... 367 12.4 Environmental and Sustainability Factors ............................ 368 12.4.1 Greenhouse Gas Emissions...................................... 368 12.4.2 Biodegradable and Recyclable Plastics .................... 368 12.5 Feasibility for Industrial Purposes in the Near Future.......... 370 12.6 Current Prospects and Market Potential of Biopolymer-Based Products .................................................. 372

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Contents

12.7

12.6.1 Production Capacities............................................... 372 12.6.2 Geographical Regions .............................................. 372 12.6.3 Industry Expansion................................................... 373 Conclusions............................................................................ 374

Index...................................................................................................................... 377

Preface Biopolymer and Biopolymer Blends: Fundamentals, Processes and Emerging Applications encompasses a retrospective of the classifcation and the chemistry of biopolymers, including natural biopolymers, synthetic biopolymers, and biodegradable polymers from biomass and marine-based resources. This book aims to provide a comprehensive framework to show the possible utilization of biopolymers in biopolymer blends and bionanocomposites as an alternative source to the conventional non-biodegradable polymers from petroleum-based sources. On top of that, this book describes past and recent developments and applications of biopolymers and their blends in 3D printing and other emerging applications towards the sustainable, intelligent, and high-end applications such as packaging, pulp and paper, agriculture, biomedical, cosmetic, and marine industries. Chapters include several studies and the latest applications in similar felds to show the present and future potential of biopolymers and their blends produced in the advancement of biomaterial technology. Topics range from the various approaches explored and developed for biopolymers and biopolymer blends to their properties in tackling environmental, performancerelated improvements, challenges, and factors of the increased use of biocomposites in various industries. The book also discusses the recent developments of biopolymers and biopolymer blends in the processes, applications, and challenges in various highimpact research and applications. The fundamentals of biopolymers and biopolymer blends from natural and synthetic sources and the synthesization process, characterizations, desired performance for specifc applications, and current challenges and future prospects for biopolymers and biopolymer blends are also discussed. This book covers the following topic: (1) biopolymer composites: processing, surface modifcation, and characteristics; (2) biodegradation and composting of biopolymers; (3) state-of-the-art natural biopolymers for bionanocomposites; (4) biopolymers in 3D printing technology; (5) applications of biopolymer blends and biopolymer-based nanocomposites; (6) starch-based flm with essential oils for antimicrobial packaging; (7) chitosan-based chemical sensors: sensing mechanisms and detection capacity; (8) seaweed-based biopolymers for sustainable applications; (9) emergence of biopolymers from sugar palm starch for packaging: characteristics and performance; (10) biopolymers for drug delivery applications: modifcations and performance; (11) crosslinking networks of functional biopolymer hydrogels; and (12) current challenges and future prospects of biopolymer blends and biopolymer-based nanocomposites. This book consolidates various important subjects on biopolymers and biopolymer blends into a single resource, intended for a wide range of readers, including researchers, scientists, and students. It particularly caters to individuals involved in designing with biopolymers and biopolymer blends, exploring alternatives to traditional polymers by transitioning to biopolymers, and aiming to gain a comprehensive understanding of the potential and feasibility of biopolymers in various felds. Additionally, the book contains valuable information that can be benefcial to high school and foundation-level students interested in biopolymers and biopolymer blends. xiii

Authors Professor Datuk Ts. Dr. Abdul Khalil H.P. Shawkataly, F.I.M.M.M. (U.K.), F.I.A.A.M. (Sweden), known as Abdul Khalil, H.P.S. He completed his Doctoral Degree in the area of Biocomposites at the University of Wales, Bangor, United Kingdom. Currently, he is senior professor at the School of Industrial Technology, Universiti Sains Malaysia (USM), and has served for more than 32 years since 1992. His areas of expertise include biocomposites, biopolymers, and nanocellulose-based materials. Recently, he has been nominated and granted the International Fellow of International Association of Advanced Materials (FIAAM), Sweden, for his contributions in environmental and green nanotechnology in 2021. He was also awarded the 13th National Academic Award in Science, Category Journal Publishing, by the Malaysian Ministry of Higher Education and appointed Academic World Class Professor by the Ministry of Education and Culture, Indonesia, for 2018–2022. He has been listed and awarded as Top Research Scientist Malaysia (TRSM) from 2014 until now. His excellence in research and publication with high numbers of citation have qualifed him to be awarded Malaysia’s Rising Star Award 2015 (1% Highly Cited Paper in the World–Material Science), Thomson Reuters @ Web of Science, and Malaysia’s Research Star Award (MRSA) 2018 in the category High Impact Paper (Natural Sciences)–Elsevier/Scopus, followed by Malaysia’s Research Star Award (MRSA) 2019 in High Impact Paper (International Collaboration)–Elsevier/ Scopus from the Ministry of Higher Education, Malaysia. At the national and institutional levels, he has many achievements and has been a successful research, publication, administration, and committee head. He has also served as adjunct professor at Universiti Syiah Kuala, Indonesia, and at Universiti Pendidikan Sultan Idris (UPSI), Malaysia. He was on the Advisor Panel of the Malaysian Citation Centre, Ministry of Education; Research Fellow at Institute of Forestry and Forest Product; and on the Standards Malaysia (MS) National Committee for Wood-Based Panels. Also, he was appointed to the editorial board of six ISI/Scopus journals and reviewed more than 200 articles in reputable journals. To date, he has published over 350 scientifc articles in WoS/Scopus, with an H-Index of more than 60 and over 16,000 citations. He has also published six academic books, 50 book chapters, and four non-academic books. Dr. Nurul Fazita, M.R., received bachelor’s and master’s degree in bioresources and paper and coatings technology at the School of Industrial Technology, Universiti Sains Malaysia. She completed her doctoral degree in the area of biocomposites in 2014 at the University of Auckland, New Zealand. The topic of research for her PhD was Thermoforming of Composites Made from Bamboo Fabric and Thermoplastic Polymers. After graduation, she joined the School of Industrial Technology at Universiti Sains Malaysia as a senior lecturer. As a senior lecturer, she taught several courses, such as Statistics with Computer Applications, Fibres and Lignocellulosic Composites, and Basic Bioresources Science and Technology, as well as Bioresources Technology Laboratory. She continues to work and publish in the broader area of biocomposites. She has conducted research for various projects in the areas of thermoforming, biopolymers, natural fber composites, and 3D printing. xv

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Authors

Dr. Mohd Nurazzi N. is a senior lecturer at the School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia. Before joining Universiti Sains Malaysia, he was experienced as Post-Doctoral Fellow at the Centre for Defence Foundation Studies, National Defence University of Malaysia, under the Newton Research Grant for the study “Role of Intermolecular Interaction in Conductive Polymer Wrapped MWCNT as Organophosphate Sensing Material Structure”. He obtained a diploma in polymer technology from Universiti Teknologi MARA (UiTM) in 2009, a bachelor of science (BSc.) in polymer technology from Universiti Teknologi MARA (UiTM) in 2011, and a master of science (MSc.) from Universiti Teknologi MARA (UiTM) in 2014 under a Ministry of Higher Education Malaysia scholarship. He was awarded a PhD from Universiti Putra Malaysia (UPM) in materials engineering under a Ministry of Higher Education Malaysia scholarship. His main research interests include materials engineering, polymer composites and characterizations, natural fber composites, and carbon nanotubes for chemical sensors. To date, he has authored and co-authored more than 100 citations indexed in journals, three books, 30 book chapters, 15 conference proceedings/seminars, and three journal special issues as a guest editor on polymer composites, natural fber composites, and materials science–related subjects.

1 Processing, Surface

Biopolymer Composites Modification and Characteristics

1.1

INTRODUCTION

Since the Renaissance, human civilization has continued through the present. Through the snowball effect, each new intellectual advance leads to further advancements. Countless materials have been innovated, developed and improvised to enhance human lifestyles. Believe it or not, most of the materials are made of polymers, which are a large group of small molecules or monomers combined and linked together in a long chain (Namazi, 2017). Products made from polymers are all around us in the current world. We could be familiar with the top two materials which have been utilized extensively from the Renaissance to the modern age— plastic and rubber. Plastic and rubber have become ubiquitous today. One can spot plastic and rubber material in almost every aspect of our lives, from everyday grocery bags to parts of cars and electronic gadgets. Perhaps we may not even realize that we have used or touched plastic or rubber material in the last minute. In fact, it is not surprising that nearly all fields, including physics, textiles, pharmaceuticals, medicine, molecular biology, biochemistry, construction and mechanical and chemical engineering, have participated in research and development projects related to plastic and rubber. The polymer industry has developed speedily, even faster than copper, steel, aluminum, and other industries (Abdul Khalil et al, 2023). Human life depends much on these materials, as their impact on the present way of life is almost immeasurable. Diving into the understanding of polymers, they can be divided into two main groups, natural polymers and synthetic polymers. Polymers have been around us in the natural world since the very beginning (e.g., cellulose, starch, polysaccharides, proteins and latex), whereas synthetic polymers, which are usually produced by using carbon atoms provided by petroleum or other fossil fuels, have been exploited since the middle of the 19th century. But what is so special about polymers that they seem to be exceptionally useful to us? The term “polymer” is derived from Greek, where poly means “many” and meres means “parts”. Polymers with high molecular weights range between 10,000 and 1,000,000 g/mol or more (Abdul Khalil 2023; Barot et al., 2019) They are made of many small molecules, arrayed in a high number of repeating units called monomers. They are usually bonded together by covalent bonds. When monomers of the same or another type bond together under suitable conditions, a reaction takes place to form a long polymer chain. This is the main DOI: 10.1201/9781003416043-1

1

2

Biopolymers and Biopolymer Blends

factor that causes polymers to be durable and useful in many felds, which has transformed our modern world. However, the invention of materials has also come with a price to pay. News regarding the decrease of crude oil or petroleum has been notable as the demand for petroleum-based polymers is increasing. Globally, we consume over 11 billion tons of oil from fossil fuels every year, and crude oil reserves are dropping at the rate of more than 4 billion tons per year (Ali et al., 2017). If this continues, our oil deposits could run dry in 53 years (Kuhns & Shaw, 2018). Lately, news regarding plastic pollution has been alarming for most of us. Previous research estimated that approximately 8 million metric tons (Mt) of macroplastic and 1.5 Mt of microplastic go into the ocean yearly (Ng et al., 2018). If this issue remains unaddressed, the waste could double by 2050 (Lebreton & Andrady, 2019). Countries such as China and India have encountered white pollution due to the extensive usage of plastic in the agricultural sector. Apart from that, the toxins released from the micro-plastics are also something to note, as plastics are resilient to biodegradability. All of these have created concerns for scientists and environmentalists to be more stringent in fnding methods, alternatives and new material to mitigate environmental pollution and make our earth a better place to live. One of the new ways is to increase the use of green material and apply green technology instead of using purely synthetic material and technology that would harm the environment. In the recent years, biopolymers have been emphasized in many sectors, including pharmaceutical, medical, agriculture, building, electronic and automobile, with the ultimate goal to (1) replace existing synthetic polymers for sustainable development, (2) reduce the current struggle of decreasing crude oil and (3) counter the problem of environmental pollution. This is due to the fact that biopolymers are more environmentally friendly compared to crude oil-based polymers. However, biopolymers alone are often not suffcient to stand as an individual material. They need to be reinforced by fllers or combined with other polymers to achieve the desired performance depending on the functionality (Abdul Khalil et al., 2019). This is one of the reasons research on biopolymer blends and bionanocomposites has been surging lately. Biopolymer blends are materials formed by blending two or more biopolymers, either with a physical or chemical approach. The aim of blending is to improve or tailor properties for certain applications with added value. Nevertheless, there are other motive for blending, too. This includes cost reduction. For instance, poly(lactic) acid (PLA) and starch is one of the most common biopolymer blends, as PLA is able to contribute mechanical and water barrier properties, while starch helps to reduce the material cost. Another case is found in polyhydroxyalkanoates (PHAs), which are versatile yet expensive. Hence, they are often blended with starch to reduce the material cost (Imre & Pukánszky, 2013). A biocomposite is defned as a material made from the combination of two or more distinct constituents to obtain a newly improved material, and a bionanocomposite is defned as biohybrid material composed of biopolymers and an inorganic moiety, showing at least one dimension on the nanometer scale (1 nm = 10 −9). However, bionanocomposites are favored over biocomposites, as nanotechnology is growing more popular. They are said to be the materials of the 21st century owing to their uniqueness in applications that is not found in the usual composites. Such unique applications include smart food

Biopolymer Composites

3

packaging and tissue engineering in regenerative medicine, electronics and sensors, drug delivery, gene therapy, cosmetics and many others. Almost every sector of industries associated with polymers has begun to venture into nanoscale materials. This motivates materials scientists to further develop and augment bionanocomposites for targeted fields. The two main benefits that researchers pay attention to in the development of bionanocomposites are the remarkable advantages of their synergy with inorganic nanosized solids and their biodegradability. These open the door to possibilities for diverse applications for current and future needs.

1.2

RETROSPECTIVE ON BIOPOLYMERS

Past history has shown that humans have been using natural resources for almost a thousand years. An overview of when humans began to venture into using polymer materials and some of the milestones achieved are reflected in Figure 1.1. As early as 1600 BC, ancient Mesoamericans used a natural material and turned it into balls (Tarkanian & Hosler, 2011). The first attempt at polymer science was by Henri Braconnot in the 1830s. Braconnot, along with Christian Schönbein and a team, synthesized semi-synthetic materials called celluloid and cellulose acetate from the derivatives of the natural polymer cellulose (Thakkar et al., 2020). The term “polymer” was invented by Jöns Jakob Berzelius in 1833 (Suter, 2013).

FIGURE 1.1 Review of past polymer science.

4

Biopolymers and Biopolymer Blends

In the 1840s, Friedrich Ludersdorf and Nathaniel Hayward added sulfur to latex and discovered that it was able to prevent stickiness. However, it was only in 1844 that Charles Goodyear patented the process of vulcanizing latex with sulfur and heat in the United States, while Thomas Hancock patented a similar process in the United Kingdom. This procedure enhanced natural rubber and prevented it from melting with heat without losing fexibility. This method helped to produce practical products such as waterproofed articles and rubberized materials. Vulcanized rubber made possible the production of tires for bicycles and later for automobiles made by the Goodyear Tire Company (Princi, 2011). In 1884, an artifcial fber plant based on regenerated cellulose, also known as viscose rayon, was used as a substitute for silk by Hilaire de Chardonnet. However, it was extremely fammable (Shabbir & Mohammad, 2017). The key breakthrough in polymer science happened in 1907 when a Belgian-American chemist called Leo Baekland invented Bakelite—a fully synthetic plastic (thermosetting phenol-formaldehyde resin) without any trace of natural molecules. The success of Baekland led many major companies to invest in the work of research and development of new polymers. Regardless of the advancement of polymer synthesis at that time, the nature of polymers was only known through association theory or aggregate theory by Thomas Graham in 1861, where he hypothesized that polymers were colloids and aggregates of molecules, connecting with each other via unknown intermolecular forces. It was not fully understood until 1922 when Hermann Staudinger (a professor of organic chemistry at the University of Applied Sciences in Zurich) suggested that polymers encompassed long chains of atoms bonded together by covalent bonds. His research was to pioneer modern manipulations of both natural and synthetic polymers. He stated two terms that are key to understanding polymers: polymerization and macromolecules. After only a decade, Staudinger’s work was acknowledged and gained acceptance. In 1953, he was awarded the Nobel Prize for his work (Pathak et al., 2014). However, the commercial polymer industry only surfaced during World War II due to limited resources and supply of natural materials, including silk and rubber. This instigated the increased production of synthetic materials such as nylon and man-made rubber and advanced polymers such as Kevlar and Tefon (Pathak et al., 2014). In 1954, polypropylene was invented by Giulio Natta and then manufactured in 1957 (Sivaram, 2017). As time has passed, these materials have continued to invigorate the growth of the polymer industry and materials from natural polymers to synthetic polymers and now biopolymers. Demand for biopolymers has been growing rapidly since the last decades, and it is even more noticeable in recent years as the signifcance of biopolymers has been realized once again. This is because many of us have begun to note that petroleum resources have caused various negative environmental impacts and petroleum resources are not infnite. Fossils and crude oil will eventually run out in the future. Therefore, there is an urgency to fnding alternative ways to solve the current issues for a sustainable future. However, what are biopolymers, and why do they attract more attention than fossil fuel-based materials today?

Biopolymer Composites

1.3

5

DEFINITION OF BIOPOLYMERS

The term “biopolymers” has always held ambiguity, as it is often associated with terms such as “biobased” or “biodegradable”. According to ASTM D6866-12 and ASTM D7026-13, a biobased polymer is defned as a polymer that comes from renewable sources whereby the carbon content can be determined by the number of carbon traces released from the short CO2 cycle. Biodegradable polymers, on the other hand, are defned as polymers that are able to biodegrade in the soil via the action of microorganisms and release carbon dioxide and water as their end products. Biodegradable polymers are legally certifed by ISO 17088:2012 or ASTM D6400-12 standards. In short, biobased polymers are important for their raw material or resources, while biodegradable polymers are important for their functionality (Niaounakis, 2015a). In order to reduce confusion, biopolymers have been clearly defned based on three major categories (Hassan et al., 2019), as follows. Biopolymers from renewable sources that are biodegradable • Defnition: The frst category, biopolymers from renewable sources, can be defned as a kind of polymers composed of repeating monomers that are produced from living beings (Hassan et al., 2019). • Source: This category of biopolymers can be obtained from naturally occurring plants, algae, animals or microorganisms; synthesized chemically from biological starting compounds such as starch, corn and sugar; and produced by fermentative biotechnological processes from microorganisms (Niaounakis, 2015a). • Examples: Examples of biopolymers from this category vary from polysaccharides, protein, lignin and chitosan-based plastics to PLA to PHAs (El-Hadi, 2018; Varma & Gopi, 2021). • Further remarks: These polymeric biomolecules are formed by monomeric units bonding together in covalent bonds. The predominant characteristic that makes biopolymers more environmentally friendly than petroleum-based material is their ability to biodegrade naturally in the environment. Generally, they do not release pollutants but only CO2 and water in the case of hydrogen, carbon and oxygen compounds (Chen et al., 2016). Biopolymers from renewable sources that are not biodegradable • Defnition: The second category is biopolymers made of biomass or renewable resources that are not biodegradable (Hassan et al., 2019). • Source: The biopolymers under this category are produced from using biomass or renewable sources, such as bioethanol from sugarcane, polyamide 11 from castor oil or specifc biopolyesters (Niaounakis, 2015a). • Examples: Examples of existing biopolymers are biopolypropylene (bioPP), biopolyethylene (bio-PE or green PE) and biopoly(vinyl chloride) (bioPVC) (Reichert et al., 2020).

6

Biopolymers and Biopolymer Blends

• Further remarks: These biopolymers are produced using the same method as petroleum-based PEs. The only difference between these biopolymers and petroleum-based PE is the source used. They are said to emit fewer greenhouse gases and be more energy effcient compared to petroleumbased PEs (Reichert et al., 2020). Biopolymers from non-renewable sources that are biodegradable • Defnition: The third category is biopolymers that are produced from certifed biodegradable and compostable synthetic crude oil-based sources; some are from biodegradable “aliphatic-aromatic” copolyesters (Hassan et al., 2019). • Source: Can be derived fully from fossil fuels and/or combined with biobased materials such as polylactic acid, starch and so on (Abdelrazek et al., 2016). • Examples: Examples of these biopolymers are polycaprolactone (PCL), poly(butylenes succinate) (PBS), polybutylene adipate-terephthalate (PBAT) and polyvinyl alcohol (PVOH) (Abdelrazek et al., 2016; Lamnawar et al., 2018; Lins et al., 2015). • Further remarks: The polymer structure contains chemical groups that can be easily broken down by the action of microorganisms. They are ideal for combination with other biodegradable polymers that have high modulus and strength but are very brittle. Some even have water-soluble properties (e.g. PVOH) (Lamnawar et al., 2018). Rajeshkumar (2021) defned “biopolymers” as polymers that are derived from renewable resources and biological and crude oil-based biodegradable polymers. From the three categories, the frst category of biopolymers seems to be the most environmentally friendly, as they are obtained from sources that can be regenerated or grown every year, and they are able to biodegrade via the decomposition action of microorganisms, which eventually returns the material to the environment in a natural way. These are followed by the second category, which is biodegradable even though they are obtained from non-renewable sources. Nevertheless, in recent years, the term “bioplastic” has conventionally been used due to the increase of day-to day usage of plastics. Plastics, like polymers, are a subset of polymers, whereas bioplastics are thermoplastics made from biopolymers from biobased sources such as sugar, seaweed or starch. Biopolymers are a diverse category of materials that include not only bioplastics but also natural polymers like silk, chitosan and fur. For example, PLA is both a bioplastic and a biopolymer. According to European Bioplastics, a bioplastic is a plastic material that is either biobased, biodegradable or has both properties. To put it another way, 100% biobased plastics can be non-biodegradable, depending on the process applied to produce the bioplastic, while 100% of petroleum-based plastics can be degradable due to their chemical structure, which can be biodegraded by microorganism. Figure 1.2 depicts common types of biopolymers and how they are classifed based on biodegradability and biobased content.

Biopolymer Composites

FIGURE 1.2

7

Biopolymer material coordination chart.

Nevertheless, the preference for using natural biopolymers has increased. The advantages of using biopolymers for the aim of sustainable manufacturing are critical for: • Resource efficiency • The resources being cultivated on (at least) an annual basis • The cascade use principle, as natural biopolymers can first be used for materials and then for energy generation • Lowering the carbon footprint and greenhouse gas emissions of materials and products, which eventually conserves fossil resources

1.3.1

ReseaRch TRends foR BiopolymeRs

In view of the research trends for the past decade, interest in biopolymers has grown, as the number of scientific publications in the field of biopolymers has increased from 2009 to 2023 (over 70,000 publications), as shown in Figure 1.3. Furthermore, biopolymers had the highest number of publications among nine other fields of study from 2009 to 2023, over 10,000, including materials science, chemistry, chemical engineering, chitosan, polymers, nanotechnology, composite material, organic chemistry and nuclear chemistry. The growth of publications in recent years reflects

8

Biopolymers and Biopolymer Blends

FIGURE 1.3 The trend of publications on biopolymers from 2009 to 2023. (Data extracted from Lens.org.)

the significance of biopolymer innovation, research and development in modeling, processing, synthesizing and production.

1.3.2

maRkeT TRends foR BiopolymeR-Based pRoducTs

Beyond the research field, the global biopolymer market has also been experiencing an increase curve, and it is forecast that the biopolymer market will undergo outstanding growth, with a compound annual growth rate (CAGR) of 21.7% (from USD 10.5 to 27.9 billion) from the year 2020 to 2025, as reported by Bioplastics and Biopolymers Market. The data was collected based on the different regions: Asia Pacific (APAC), Europe, North America and the rest of the world (RoW), as indicated in Figure 1.4. As observed from the trends of research and the global market, it can be deduced that biopolymers have become increasingly important in society in today’s world. The growing demand for biopolymer-based materials in the market could be mainly due to several reasons, such as the increasing adoption of green consumerism among consumers as more consumers begin to be aware of the need to reduce the usage of non-renewable resources, the uncertainty about future resources of the petrochemical industry, the inutility of recycling over the years, the ability to use waste as a substance for biopolymer production and the recent use of biopolymers in the medical and pharmaceutical industries (Hojnik et al., 2019). Therefore, it is crucial to emphasize research on biopolymer-based materials to make them functional, practical and economically viable to replace conventional polymers used today.

1.4 BIOPOLYMER BLENDS AND BIONANOCOMPOSITES In the research field, biopolymer blends and bionanocomposites have been clearly mentioned and studied, particularly in the recent years. In fact, these two terms are

Biopolymer Composites

9

FIGURE 1.4 Bioplastics and biopolymers market by region from 2018 to 2025. (Adapted from Bioplastics and Biopolymers Market, April 2020).

sometimes used interchangeably. Although there is a lack of a definite definition for them, biopolymers and bionanocomposites can be briefly introduced as follows (Shaghaleh et al., 2018; Thomas et al., 2013; Vishal, 2017). • Biopolymer blends • Definition: A material in which two or more types of biopolymers are blended together (Thomas et al., 2013). • Examples of biopolymers: The common type of blending is blending a biobased polymer with a synthetic polymer (e.g. polyester, starch/ PE). Biopolymer blends can be made from a blend between two types of renewable biopolymers (e.g. starch/chitosan, seaweed/starch, etc.) or between a renewable biopolymer and a biodegradable synthetic polymer (e.g. PLA/PBAT, PLA/PCL, etc.) • Bionanocomposites • Definition: Sometimes known as “nanobiocomposites”, “green composites” or “biohybrids”. Bionanocomposites are nanocomposites made of a mixture of naturally occurring biopolymers and inorganic moiety. The most prevalent part is the nanostructure of this material. • Examples of bionanocomposites: They are made of a main matrix from biopolymers, which gives the shape, main function and structural organization, and at the same time are incorporated with nanoparticles, which tunes the structure, value, properties and functionality of the entire composite system. These nanoparticles can serve to improve specific functions of the composite for a particular application.

10

Biopolymers and Biopolymer Blends

Regardless of their dissimilarities in the concept and materials used to fabricate them, both biopolymer blends and bionanocomposites share common aims today, which are (Haghighi et al., 2021; Varma & Gopi, 2021): • To reduce and save the cost of a material. • To have certain added-value properties of a material such as impact strength, thermal stability, permeability, melt strength, antioxidants, electrical conductivity and antimicrobial properties. • To widen the scope of usage and application of a material. • To change and increase degradation rates for better biodegradability. Biopolymer blends and bionanocomposites have been studied in a wide range of approaches including biomedical, food technology, packaging, electronics, tissue engineering and so on. In fact, many documents and publications have reported on their advantages and potential characteristics to replace non-biodegradable crude oil-based plastics. This is further evaluated through the trend in publications on biopolymers and bionanocomposites in the next subsections.

1.4.1 RESEARCH TRENDS FOR BIOPOLYMER BLENDS AND BIONANOCOMPOSITES In the past decade, the trend of biopolymer blends has also increased, because demands on biopolymer-based materials and concern for the environment have both increased. To date, research on biopolymer blends for different applications is still actively conducted, as seen in the chronological events listed in Table 1.1. Biopolymers that are gaining popularity to fabricate biopolymer blends include those directly or indirectly obtained from renewable resources such as PLA, starch, chitosan, alginate, carrageenan and cellulose. Research on bionanocomposites continued to expand when Watzke and Dieschbourg (1994) reported a sol-gel method to fabricate a novel silica-biopolymer nanocomposite using different biopolymers such as gelatin and chitosan. From this study, the system for sol-gel studies under low or microgravitational conditions was developed by comparing the structural features between silica–gelatin bionanocomposites with silica–biopolymer composites of nongelling biopolymers in microemulsions (e.g., chitosan). Since then, research on bionanocomposites began to emerge widely, and applications in diverse felds can be seen, especially for foods, pharmaceuticals and so on. In the 1990s, research onbionanocomposites inclined to the application of packaging. This could be due to the effort made by researchers to counter the environmental problems caused by conventional plastics. In a nutshell, natural polymers were once explored by humans a long time ago. However, due to the success of human-made fully synthetic petroleum-based polymers and the rapid commercialization of the materials, humans begin to rely on them, only to realize later that overusage of the materials led to the scarcity of crude oil and deterioration of our environment. Considering the growth of the population, the usage of materials will increase in the years to come. Therefore, it is essential for scientists and industrialists to fnd more alternatives to curb the problems mentioned by enlarging the resource base, fnding ways to use existing raw materials,

11

Biopolymer Composites

TABLE 1.1 Chronological Events for Biopolymer Blends Year

Remarks

1977

Starch and ethylene-acrylic acid copolymer Permeability barriers by controlled morphology of polymer blends Starch–gum blends Hydrogel–melanin blends Collagen/poly (vinyl alcohol) blends Poly (ethylene terephthalateco-diethylene glycol terephthalate) and polyethylene oxide blends Polyol-plasticized pullulan– starch blends Microencapsulation of biopolymer blends (gum arabic, mesquite gum and maltodextrin) Starch-based blends Arabic gum, mesquite gum and maltodextrin DE 10 blends Caseinate–pullulan bilayers and blends DNA-conductive polymer blends Collagen/biopolymers Lactide and PLA biopolymers Bis(pyrrolidone-4-carboxylic acid)-based polyamides Gelatin–chitosan blend flms

Packaging

(Otey et al., 1977)

Packaging

(Subramanian, 1985)

Food products Ocular devices Biomedical Implanted biomaterial

(Bielskis et al., n.d.) (Chirila et al., 1992) (Sarti & Scandola, 1995) (Barcellos et al., 1998)

Food products

(Biliaderis et al., n.d.)

Food technology

(Pedroza-Islas et al., 2000)

Mulching Food products

(Halley et al., 2001) (Pérez-Alonso et al., 2003)

Food packaging Transistors

(Kristo & Biliaderis, 2006) (Ouchen et al., 2008)

Tissue engineering Drinking cups Compatibilizer

(Sell et al., 2009) (Groot & Borén, 2010) (Ayadi et al., 2013)

Packaging

Alginate/keratin blend Kappa-carrageenan and cellulose derivatives Poly (butylene-adipate-coterephtalate)/poly (lactic acid) (PBAT/PLA) PCL/polymethyl methacrylate (PMMA) biopolymer blends

Biomedical Green polymer electrolyte

(Benbettaïeb et al., 2014) (Gupta & Nayak, 2015) (Rudhziah et al., 2015)

Compatibilizing agents

(Lins et al., 2015)

Medical applications and some optical systems

(Abdelrazek et al., 2016)

1985

1989 1992 1995 1998

1999 2000

2001 2003

2006 2008 2009 2010 2013 2014 2015 2015 2015

2016

Fields/Applications

Ref.

(Continued )

12

Biopolymers and Biopolymer Blends

TABLE 1.1 (Continued) Chronological Events for Biopolymer Blends Year

Remarks

2016

Biomimetic nanofbrillation in two-component biopolymer blends Pectin and soy protein blends

Transportation and energy-related applications Meat replacers

Biopolymer-prebiotic carbohydrate blends Protein blends

Additive

Blend of organic semiconductors and biopolymers Nanostructured cellulosexyloglucan blends Agar, gelatin and wax blends Plasticized starch/zein blends Polylactic acid and polybutyrate adipate terephthalate blends Polyhydroxybutyrate (PHB) blends Soy protein–agar blends

Printable and fexible phototransistors

2016 2016 2016 2017

2017 2017 2017 2018

2018 2018 2018 2018

2018 2018 2019 2019 2019 2019 2019 2019

Cellulose, chitosan, starch and gelatin blends Aliphatic-aromatic copolyester and chicken egg white fexible biopolymer blend Chitosan/peg blends K-carrageenan and gelatin blends Chitosan-based blend hydrogels Blends of silk fbroin and collagen Starch–pectin biopolymer blends Chitosan-based blend hydrogels Chitosan/polyvinyl alcohol (PVA) blends Polyhydroxybutyrate and polylactic acid blends

Fields/Applications

Food technology

Ref. (Xie et al., 2016)

(Dekkers, Nikiforidis et al., 2016) (Silva et al., 2016) (Dekkers, de Kort et al., 2016) (Huang et al., 2017)

Water treatment

(Huang et al., 2017)

Medicine or biotechnology Compatibilizing agent Film

(Fuchs et al., 2017) (Favero et al., 2017) (Lamnawar et al., 2018)

Biomedical

Biosensor

(El-hadi & Abd Elbary, 2018) (Rivadeneira et al., 2018) (Elhaes et al., 2018)

Food packaging

(Tiimob et al., 2018)

Injectable scaffolds Tissue engineering

(Lima et al., 2018) (Tytgat et al., 2018)

Wound healing

(Rasool et al., 2019)

Hair care, cosmetics Material conservation

(Grabska & Sionkowska, 2019) (Y & Rao, 2019)

Wound healing

(Rasool et al., 2019)

Burn wounds

(Bano et al., 2019)

Drug delivery systems

(Harting et al., 2019)

Wound dressing

13

Biopolymer Composites Year

Remarks

2019

Polylactic acid/poly(D,L-lactic acid) (PDLLA)/ polyhydroxybutyrate blends Poly (vinyl alcohol) flms enriched with tomato Two-step blending in the properties of starch/chitin/ polylactic acid Sodium alginate–assam bora rice starch–based multiparticulate system containing naproxen Biodegradable and antimicrobial polylactic acid–lactic acid oligomer (PLA–OLA) blends containing chitosan-mediated silver nanoparticles with shape memory Xanthan–curdlan nexus Molecular modeling analyses for modifed biopolymers Biopolymer blends of polyhydroxybutyrate and polylactic acid Graphene-based biopolymer TiO2 electrodes using pyrolysis Konjac glucomannan/alginate flms enriched with sugarcane vinasse Effect of empty fruit bunches microcrystalline cellulose (MCC) on the thermal, mechanical and morphological properties of biodegradable poly (lactic acid) and polybutylene adipate terephthalate composites The role of biopolymer-based materials in obstetrics and gynecology applications

2020 2020

2020

2020

2020 2020 2020

2020

2020

2020

2021

Fields/Applications

Ref.

Biodegradable optical fbers

(El-Hadi, 2018)

Active packaging

(Szabo et al., 2020)

Biomedical

(Olaiya et al., 2020)

Anti-infammatory drug

(Sarangi et al., 2020)

Medical

(Sonseca et al., 2020)

Edible packaging HIV protease inhibitors

(Mohsin et al., 2020) (Omar et al., 2021)

Packaging

(Aydemir & Gardner, 2020)

Photo-electrocatalysis in water treatment process

(Kaur et al., 2020)

Mulching

(Santos et al., 2020)

Packaging

(Nor Amira Izzati et al., 2020)

Biomedical

(Jummaat et al., 2021)

(Continued )

14

Biopolymers and Biopolymer Blends

TABLE 1.1 (Continued) Chronological Events for Biopolymer Blends Year

Remarks

2021

Isolation of textile waste cellulose nanofbrillated fber reinforced in polylactic acid-chitin biodegradable composite for green packaging application Recent progress in modifcation strategies of nanocellulose-based aerogels for oil absorption application Insights into the role of biopolymer-based xerogels in biomedical applications Cinnamon-nanoparticle-loaded macroalgal nanocomposite flm for antibacterial food packaging applications Coffee waste macro-particle enhancement in biopolymer materials for edible packaging

2022

2022

2023

2023

Fields/Applications

Ref.

Packaging

(Rizal et al., 2021)

Absorbent

(Iskandar et al., 2022)

Biomedical

(Abdul Khalil et al., 2022)

Packaging

(Rizal, Abdul Khalil, Abd Hamid et al., 2023)

Packaging

(Rizal, Abdul Khalil, Hamid et al., 2023)

converting waste materials into something useful and producing new materials out of the resources which are abundantly available in nature. Alongside the advancement of technology, the potential of biopolymers can be unleashed, and current biopolymer materials, including biopolymer blends and bionanocomposites, could be used as new-age materials. Nevertheless, how well can these materials perform, what are they made of, how are they fabricated and how sustainable are they? Follow through the chapters to discover food for thought and the science behind these questions.

1.4.2 PROS AND CONS OF BIOPOLYMER BLENDS AND BIONANOCOMPOSITES Food packaging, medicine, pharmaceuticals and cosmetics all make extensive use of biopolymers. Natural biopolymers especially contain high nutritive value, and they can be used as a food ingredient. Applications such as using chitosan to improve processes such as drug delivery and tissue regeneration, and modern technologies, such as 3D printing, make biopolymers a candidate for a wide range of applications (Diyana et al., 2021). Biopolymer blends and bionanocomposites share common pros and cons, as shown in Figure 1.5.  Although biopolymers show considerable functional properties such as mechanical, thermal and water barriers and could be a good candidate to replace conventional

Biopolymer Composites

15

FIGURE 1.5 Pros and cons of biopolymer blends and bionanocomposites. Reproduced from Diyana et al. (2021).

petroleum-derived plastics in a wide range of applications, they still lack some functionality, especially in durability and shelf life compared to petroleum-based plastics that are used conventionally. In short, biopolymer blends and bionanocomposites have two main advantages over synthetic plastics in terms of practicality: biodegradability and/or compostability and availability from renewable resources. When it comes to commercial interests, the latter appears to be more important, which is blending with non-degradable biopolymers such as bio-polyethylene or bio-polypropylene derived from sugar cane. This is because of the process and final properties, which make them similar to conventional durable plastic. Furthermore, they must be sorted out from conventional plastic when it comes to recycling (Di Bartolo et al., 2021). Another challenge encountered is the use of bionanocomposites with nanofillers in areas such as food packaging. This has been an issue that has raised concerns about nanoparticle migration and the toxicological properties of the associated organomodifiers (Istiqola & Syafiuddin, 2020; Souza et al., 2013). Hence, research is still needed to mitigate these challenges and to improve their properties.

1.5

NATURAL BIOPOLYMERS

Natural biopolymers can be obtained via biological systems from living organisms such as plants, animals and microbes. They are formed naturally during the growth cycle of living organisms. Biopolymers play a crucial role in the growth cycle of an organism mainly to support complex metabolic and cellular activities. The synthesis of biopolymers generally involves enzyme-catalyzed reactions and

16

Biopolymers and Biopolymer Blends

reactions of chain growth from activated monomers, which are generated inside the cell wall, cytoplasm, organelles, cytoplasmic membrane and the surface of cells and sometimes can be generated through extracellular enzymatic processes. The general biological role of natural biopolymers includes preservation of genetic information; expression of genetic information; catalysis of reactions, energy or other nutrients; protecting against the attack of other cells; sensing of biotic and abiotic factors; storage of carbon; and adhesion to surfaces of other organisms (Aggarwal et al., 2020). There are in fact pros and cons of biopolymers (Figure 1.6). Natural biopolymers that are generated naturally from natural sources such as plants, animals and microbes are readily degradable by enzymatic action from microorganisms, and the by-products of their biodegradation are water, carbon dioxide CO2 and some organic matter, which is one of the advantages of being eco-friendly. Moreover, natural biopolymers are abundant in resources, chemically inert, nontoxic and less expensive than synthetic ones; reduce CO2 emissions; and conserve fossil resources (Gowthaman et al., 2021). However, some disadvantages such as inferior thermal and mechanical properties can be encountered by natural biopolymers (Aggarwal et al., 2020; Babu et al., 2013). Therefore, natural biopolymers are often incorporated with

FIGURE 1.6

Advantages and disadvantages of natural biopolymers.

Biopolymer Composites

17

fllers or blended with other polymers to enhance their properties and to extend their application.

1.5.1 PLANT/ALGAE-BASED POLYSACCHARIDES Plant/algae-based polysaccharides are high molecular weight compounds that bind similar or different monosaccharides together by glyosidic bonds (Ma et al., 2017; Naqash et al., 2017). Plant polysaccharides in particular are usually found in the cell walls of plants. Cellulose, starch, pectin, carrageenan, agar and alginate are among the most abundant biopolymers found on earth that offer possibilities for producing environmentally friendly materials owing to their unique characteristics of being renewable, non-toxic and biodegradable. These promising characteristics enable them to be used in various applications, as displayed in Table 1.2. Cellulose polysaccharides can be found mainly in the cell wall of the plant, and by content are generally more than 60% of the dry mass of wood (Aggarwal et al., 2020). They are usually insoluble, odorless and rigid in structure, suitable to apply in wound dressing, scaffolds, thickener, wrappers, adhesives, dispersing agents and in drug delivery. However, some fruits such as apples consist of less than 10% cellulose in the cell wall, as they mostly contain pectin polysaccharides (Naqash et al., 2017). Besides the cell wall, pectin can be found in dried citrus peels or apple pomace. The unique feature of pectin is that it has the ability to form gel when it is heated. It is physically coarse to fne, white to yellowish and odorless and has a mucilaginous taste. Previous research has shown its application for antimicrobial action, antiinfammatory and as a thickening and gelling agent for jellies and jams. On the fip side, starch is not found in cell walls but in roots, bulbs, tubers or seeds of green plants, with the main function as energy storage for the plant. Staple foods like cassava, potato, rice, corn and wheat are rich in starch. Unlike cellulose, starch is soft and fexible in structure, although it has a similar physical appearance to cellulose and is odorless, tasteless and insoluble in water. It has been used in numerous applications such as a thickening agent for puddings; emulsifer, stabilizer and fat replacer in food technology; lubricant in oil boring; and fller in pharmaceutical items. In recent years, polysaccharides such as alginate, agar and carrageenan have been among the promising algae-based polysaccharides that have been of interest in the era of bioplastics compared to traditional methods of utilizing sources from corn and potatoes. This is due to primarily to advantages such as signifcantly higher productivity, which can be up to 50 times greater than that of conventional startch-based plants; the ability to thrive in a variety of environments; being a non-food resource (in Western countries); complementing terrestrial biomass production; not requiring arable land; and the fact that certain marine plants like seaweeds can fourish in saline water and be cultivated offshore without the need for fertilizers or pesticides (Priyan Shanura Fernando et al., 2019; Usman et al., 2017). Natural plant polysaccharides are biologically synthetized by plants for energy storage and structural support, whereas, conventionally, plant polysaccharides are extracted through hot water, high-concentration alcohol, alkaline solvent, enzymatic hydrolysis, ultrasonic, microwaves or even gel column chromatography (Ren et al., 2019). Plant proteins can be another alternative source of biopolymers besides plant

18

TABLE 1.2 The Sources, Physical Features and Applications of Plant/Algae Polysaccharides Plant/Algae Polysaccharide

Source

Physical Features

Applications

Ref.

Wound dressing, scaffolds, thickener, wrappers, adhesive, dispersing agent, drug delivery, packaging Thickener, stabilizer, water retention agent, adhesive, excipient, additive, packaging, mulch flm Antimicrobial action, anti-infammatory Removes metals, gelling agent, thickening agent, stabilizer, packaging Thickener, stabilizer, tablet excipient, encapsulating agent for drugs, scaffold material, wound dressing, foliar spray, packaging Laxative, thickener, stabilizer, emulsifer, coagulator, capsules, surgical lubricants, packaging Injectable vehicle for tissue engineering, drug delivery, wound dressing; gelling agent; stabilizer of aqueous mixtures, dispersions and emulsions; packaging

(Abdul Khalil et al., 2020; Aggarwal et al., 2020) (Syuhada et al., 2018; S. Wang et al., 2015)

Plant tissue, cell wall

Water insoluble, odorless, rigid

Starch

Plant seeds, tubers, bulbs, roots

Pectin

Carrageenan

Plant cell wall, citrus peels, apples pomace Red seaweeds

White, tasteless, odorless, insoluble in cold water or alcohol Coarse to fne, white to yellowish, odorless, mucilaginous taste White to slight yellowish, tasteless, odorless, heat-reversible

Agar

Red seaweeds

White, tasteless, odorless, heat-reversible

Alginate

Brown seaweeds

White to cream, odorless, tasteless, heat-reversible

(Aggarwal et al., 2020; Naqash et al., 2017) (Aggarwal et al., 2020; Cunha & Grenha, 2016) (Aggarwal et al., 2020; Cunha & Grenha, 2016) (Aggarwal et al., 2020; Cunha & Grenha, 2016)

Biopolymers and Biopolymer Blends

Cellulose

19

Biopolymer Composites

TABLE 1.3 The Source, Physical Features and Applications of Plant/Algae Proteins Plant/Algae Proteins

Source

Physical Features

Soy

Crushed or defatted soybean fakes

Sensitive to water, odorless, rigid

Zein

Endosperm of maize

Gluten

Endosperm of wheat

Coarse to fne, white to yellowish, odorless, mucilaginous taste, soluble in aqueous alcohol Chalky favor, stringy mouthfeel

Applications Wound dressing, scaffolds, thickener, wrappers, adhesive, dispersing agent, drug delivery Antimicrobial action, anti-infammatory Remove metals, animal feed Bread products, imitation meat, stabilizer, food packaging

Ref. (Aggarwal et al., 2020; Hassan et al., 2019)

(Aggarwal et al., 2020)

(Biesiekierski, 2017; Sharma et al., 2017)

polysaccharides. There are many types of plant proteins, and some of those common proteins are listed in Table 1.3. Among them are soy, zein and gluten proteins. Protein biopolymers derived directly from plants have several advantages, including being safe, low-cost, rapidly deployable, natural, renewable and biodegradable, similar to polysaccharides. Soy protein is one of the most economical proteins that can be obtained from soy beans. It can be extracted from either crushed or defatted soybean fakes that have been removed by dissolving in hexane, followed by dissolution, precipitation and acidifying processes. Soy protein has been a choice of industry for protein isolation due to its high soy protein isolate at approximately 90%, which is free from lipids and carbohydrates. Unfortunately, this limits the application of soy protein, as protein is sensitive to water and hydrophilic in nature. Hence, it is suitable to be used for biomedical applications. Nevertheless, many studies have co-blended soy protein with other biopolymers such as polylactic acid to improve its properties and reduce hydrophilicity. Zein is the predominant protein found in the endosperm of maize and possesses unique solubility in aqueous alcohol solutions. Zein is considered a waste protein, and it is not an ideal protein for humans due to its poor solubility and imbalanced amino acid profle (Shukla & Cheryan, 2001). However, it can be used as encapsulation for food and drugs. It is also applied in other areas such as textiles, plastics, coatings and adhesives (Garavand et al., 2022). Gluten can be obtained from the by-products of wheat after washing to remove starch granules and water soluble compounds. Gluten is composed of hundreds of protein monomers or oligopolymers, with two water-insoluble proteins, glutenins and gliadins (Biesiekierski, 2017). Wheat gluten has been used for flm forming for edible coating and food products due to its elasticity (Sharma et al., 2017). Due to the restriction on the resources of petroleum, biodegradable materials produced from

20

Biopolymers and Biopolymer Blends

green plants or marine feedstock are becoming more signifcant these days. Because of their abundant resources, low cost and good biodegradability, biopolymers from this particular group have been used extensively for diverse applications, be it in the industry or in the research feld.

1.5.2

ANIMAL-BASED POLYSACCHARIDES

Animal-based polysaccharides are gaining considerable interest and have been widely utilized, especially in the biomedical, pharmaceutical and cosmetics felds. There are plenty of other types of animal-based polysaccharides. However, this chapter is restricted to chitosan and hyaluronic acid, as they are among the most studied and common types of animal-based polysaccharides. Chitosan, characterized as a functional derivative of chitin, is a linear polysaccharide typically produced through the partial or full deacetylation of chitin using sodium hydroxide (NaOH) generally extracted from shells or crustaceans such as shrimp, lobster and crabs. Usually it appears in a pale, white faky form. Chitosan is usually obtained through a series of processes from cleaning to demineralizing to deproteinizing and deacetylation (Mohamed et al., 2020). Chitosan is attractive to use in cosmetics, wound dressing, food packaging, water treatment, textiles, pharmaceuticals and scaffolding owing to its non-toxicity, biocompatibility, biodegradability and anti-microbial and antiinfammatory properties (Table 1.4).

TABLE 1.4 The Source, Physical Features and Applications of Animal Polysaccharides Animal Polysaccharides

Source

Physical Features

Applications

Ref.

Cosmetics, wound dressing, paper processing, food packaging, seed coating, plant growth regulator, protein waste recovery, anti-bacterial, focculating agent, drinking water purifcation, drug delivery Cosmetics, drug delivery, wound healing, viscosity agent, medication fller, antibacterials

(Jiménez-Ocampo et al., 2019) (Aggarwal et al., 2020)

Chitosan

Shellfsh and crustacean waste materials

Pale, white and faky

Hyaluronic acid

The umbilical cord of newly born children, rooster combs, fermentation broths of streptococcus

Transparent, viscous fuid or white powder

(Fallacara et al., 2018; Graça et al., 2020)

21

Biopolymer Composites

Hyaluronic acid, also known as hyaluronan, is a type of polysaccharide, which is widely found in the extracellular matrix of animals such as such as in rooster crowns, vitreous bodies, umbilical cords, synovial fuid, and skin, although it can be produced synthetically from plants and through fermentation of microorganisms. One of the unique features of hyaluronic acid is the ability to retain water (Graça et al., 2020). Thus, many cosmetics companies use hyaluronic acid in their cosmetic products (Sakulwech et al., 2018). Other applications of hyaluronic acid include preparation of gels for drug delivery, steroid delivery, wound healing, lubricant for osteoarthritic joints, pain relief, enhanced mobility and dermal regeneration (Al-Sibani et al., 2017). Apart from animal-based polysaccharides, animal-based proteins such as collagen, gelatin and keratin have been extensively applied in food and pharmaceutical industries, as shown in Table 1.5. Collagen is the major structure of protein found in animals and consists of 20–30% of the total body proteins (Yoon et  al., 2020). It is sourced from fbroblasts and various vertebrates within the body. For example, fbrillar collagen, the most abundant type in vertebrates, assumes a vital role by contributing to the molecular architecture, shape, and mechanical characteristics of tissues. For instance, it enhances tensile strength in the skin and bolsters resistance to traction in ligaments.

TABLE 1.5 The Source, Physical Features and Applications of Animal Proteins Animal Proteins

Source

Physical Features

Collagen

Invertebrate body walls and cuticles

Hard, fbrous, insoluble protein and molecules form long, thin fbrils

Gelatin

Cattle hide, bones, fsh, pig skin, agricultural or non-agricultural

Keratin

Feathers, hair, nails, wool, horn and hooves, stratum corneum and scales

Water-soluble, translucent, favorless food ingredient, gummy when moist and brittle when dry Insoluble in most organic solvents

Applications

Ref.

Sutures, dental composites, sausage casings, pore and skin regeneration templates, cosmetics, biodegradable materials, solid support micro-carrier Stabilizer, thickener, texturizer, emulsifer, foaming, food wetting agent, pharmaceutical and medical usage, animal feed Absorbents, leather industry, drug delivery system, surgery, food industry, cosmetics, biomedical products, fertilizers, electrode material

(Aggarwal et al., 2020; Hassan et al., 2019)

(Derkach et al., 2020; Echave et al., 2017; Qiao et al., 2017) (Donato & Mija, 2020)

22

Biopolymers and Biopolymer Blends

The molecule is usually rod shaped. Collagen has been applied in tissue-based devices, drug delivery systems, wound healing, nanospheres and hydrogels due to its biodegradable and biocompatible properties, small size, large surface area, high adsorption capacity and ability to disperse in waster. Gelatin is another substantial animal-based protein that has gained interest in a wide range of applications. It can be produced through denaturation or partial hydrolysis of collagen derived from the skin, tissues and bones of animals. Due to its biodegradability, non-toxicity and cost effectiveness, it has been used widely in forming hydrogels, foaming, emulsifying, food products, animal feed and for pharmaceutical and medical properties. Similar to collagen and gelatin, keratin is also renewable, biodegradable and biocompatible in terms of its characteristics (Donato & Mija, 2020). The primary source of keratin is found in feathers, hair, nails, wool, horn and hooves, stratum corneum and scales. The unique feature of keratin is that it has a high porous network, chemical reactivity and water retention capacity, making it suitable to be utilized as a biosorbent in wastewater treatment, drug delivery systems, surgery, the food industry, cosmetics, biomedical products, fertilizers and electrode material.

1.5.3

MICROORGANISM-BASED POLYSACCHARIDES

Microbial polysaccharides constitute a form of essential commodity that is of growing interest to many industries. Their novel functions, such as consistent chemical and superior physical properties, are advantages of microbial polysaccharides over plant polysaccharides. They can be produced by microorganisms from a variety of sources. Microbial polysaccharides can be obtained from the cell wall or excreted from the cell through extracellular mechanisms and cultivated in bioreactors. Some of the optimization techniques and principles are illustrated in Figure 1.7 to show the scale-up of production of microbial polysaccharides. At the industrial scale, microbial polysaccharides are produced from the exopolysaccharides (EPSs) (i.e. the outermost structure of the microorganism) in large quantity via fermentation processes. Two main microorganisms that have gained much attention lately in producing microbial polysaccharides are fungi and bacteria (Table 1.6). This chapter covers the two polysaccharides produced by fungi, glucan and pullulan. Levan, dextran, bacterial cellulose and xanthan gum are the other main polysaccharides produced by bacteria that are also covered in this chapter. Fungi have a cell wall that consists of polysaccharides. Glucan is one of the most abundant microbial polysaccharides found in the cell wall of yeast fungi, and it functions as a regulator of innate immunity. Glucan can be either water soluble or insoluble macromolecules (Yuan et al., 2020). In general, the yeast glucan is part of Japanese food commodities, as it is mostly found in mushrooms (Złotko et al., 2019). It has also been applied in milk yielding, animal feeds, nutritional supplements, plant pests and viral invasion control, coating for surgical instruments, food packaging and cosmetics. Pullulan can be obtained from ageing stock of the polymorphic fungus Aureobasidium pullulans. It can also be synthesized from other microorganisms. Several noted microorganisms are Aureobasidium spp., Tremella mesenterica,

Biopolymer Composites

23

FIGURE 1.7 Microbial polysaccharides obtained at a large scale through cell cultivation in bioreactors. Reproduced from Wehrs et al. (2019).

Cytaria spp. and Cryphonectria parasitica, which can be discovered in backwoods soil, ocean water and plant and animal tissues (Singh & Kaur, 2019). Pullulan is highly water soluble and, due to this character by promoting oxygen barrier properties, has high moisture retention capability and is able to prevent fungal growth. These properties enable it to be used in diverse applications such as in food packaging, cosmetics and pharmaceutical industries. Besides obtaining polysaccharides from fungi, bacteria also play an essential role in producing polysaccharides. The most common polysaccharides produced from bacteria included in this chapter are levan, dextran, bacteria cellulose and xanthan gum. Levan is a class of bacteria polysaccharide excreted from the extracellular homopolysaccharides of d-fructose. Numerous bacteria are able to produce levan, including Streptococcus salivarius (i.e. a bacterium of the oral flora), Lactobacillus sanfranciscensis, Bacillus subtilis and Bacillus polymyxa, Acetobacter xylinum, Gluconoacetobacter xylinus, Microbacterium levaniformans and Zymomonas mobilis (de Siqueira et al., 2020; Hassan et al., 2019). Levans are a natural adhesive and surfactant, non-viscous and water and oil soluble. They also exhibit immunestimulating and anti-tumor properties, which are suitable in pharmaceutical and food applications as an emulsifying agent, gelling agent, surface-quality agent, encapsulation, carrier for taste and odor, photographic emulsion, molecular sieve for gel filtration and blood volume extender. Dextran is a type of exopolysaccharides known to be produced from lactic acid bacteria. Other bacteria such as the Streptococcus and Acetobacter genera have also been found to produce dextran (Ghimici & Nichifor, 2018). Dextran exhibits a flexible structure attributed to its free rotation of glyosidic bonds (Aggarwal et al.,

24

TABLE 1.6 The Source, Physical Features and Applications of Microbial Polysaccharides Microorganism Polysaccharide

Source

Physical Features

Fungal (Saccharomyces cerevisiae) cell wall

Either soluble or insoluble in water

Pullulan (fungal)

Aureobasidium pullulans

Levan (bacteria)

Bacillus subtilis, Bacillus megaterium, Bacillus cereus and Bacillus pumilus

White powder dissoluble in water; tasteless; odorless; gluey adherent solution; indissoluble in solvents such as ethanol, methanol and acetone Natural adhesive and surfactant, non-viscous and water and oil soluble

Improve milk yield, animal feeds, nutritional supplements, control plant pests and viral invasions, coating for surgical instruments, food packaging, cosmetics Inhibits fungal growth, lowviscosity fller, stabilizes the quality and texture, binder and stabilizer, protective glaze, stabilizes fatty emulsions, denture adhesive, pharmaceutical coatings Emulsifying agent helps in preparation, preservation, gelatinization, surface-quality agent, encapsulation, carrier for favor and odor, photographic emulsion, molecular sieves for gel fltration, blood volume extender

Ref. (Hassan et al., 2019; Yuan et al., 2020)

(Aggarwal et al., 2020; Singh & Kaur, 2019)

(Aggarwal et al., 2020; de Siqueira et al., 2020; Hassan et al., 2019)

Biopolymers and Biopolymer Blends

Glucan yeast (fungal)

Applications

Dextran sucrase, Leuconostoc mesenteroides, Saccharomyces cerevisiae, Lactobacillus plantarum or Lactobacillus sanfrancisco

Soluble in water and organic solvents, high viscosity

Bacterial cellulose (BC) (bacteria)

Gluconacetobacter, Pseudomonas, Rhizobium, Sarcina, Dickeya and Rhodobacter belong to the Komagataeibacter genus

Intact membranes (fber or pellets form), disassembled bacterial cellulose (BC), and BC nanocrystals (BCNC)

Xanthan gum (bacteria)

Plant pathogens such as Xanthomonas campestris

Motile, having a single polar fagellum, cream-colored powder soluble in both cold and hot water

Solidifying agent, thickening agent, improves surface quality, emulsifer in edible products, soothing, palatable, loaf mass, smoothness, storage life, cryoprotectant, viscosifer, creamy, lower synaeresis, antioxidant for food, water holding capacity, moisture content raised in non-fat mass, functional foods Packaging for foodstuffs, transparent covering, cell divider, permeable, medicine manufacturing industries, water investigation, beauty products, biocompatible, ethyl alcohol manufacturing, conducts electricity, magnetic stuff, human-made blood vessels, scaffold for tissue engineering Emulsifer and thickening agent texture, viscosity, favor release, appearance, antiseptic and water control

(Aggarwal et al., 2020; Ghimici & Nichifor, 2018)

Biopolymer Composites

Dextran (bacteria)

(Aggarwal et al., 2020; Gorgieva & Trček, 2019)

(Hassan et al., 2019; Nazarzadeh Zare et al., 2019)

25

26

FIGURE 1.8

Biopolymers and Biopolymer Blends

Gelation of dextran polysaccharide in hydrogel formation.

2020). It is highly soluble in water, biocompatible and biodegradable. Therefore, it possesses the ability to form hydrogen, as illustrated in Figure 1.8, and is normally applied in the pharmaceutical industry as a blood plasma replacement and in the food industry as a thickener for food products, to improve moisture retention and to maintain flavor and appearance. Bacterial cellulose, also known as microbial cellulose, is another type of exopolysaccharide generated from the bacteria Komagataeibacter in a carbon and nitrogenenriched media. Other bacteria that have been reported to produce bacteria cellulose include Achromobacter, Alcaligenes, Aerobacter, Agrobacterium, Azotobacter, Gluconacetobacter, Pseudomonas, Rhizobium, Sarcina, Dickeya and Rhodobacter (Rangaswamy et  al., 2015). The difference between bacterial cellulose and plant cellulose is that bacterial cellulose does not have the association of lignin, hemicellulose and pectin. Hence, it is much easier to clean and purify compared to plant cellulose. Some of its characteristics are higher water retention, longer drying time and ductility. These makes it preferable to be used in food products as dietary fiber, a thickening agent and packaging; in paper products as high-quality paper; and in pharmaceutical materials for injury and wound dressing and as an immobilizer for Lactobacillus cells (Aggarwal et al., 2020; Gorgieva & Trček, 2019). Xanthan, a bacterial polysaccharide produced from a gram-negative bacterium, Xanthmonas campestris, has been extensively studied and is commonly used as a food additive due to its viscosity and stabilizing characteristics (Hassan et al., 2019). Xanthan is found to be stable in high shear pressure, warmth and even acidic environments. It is also known for its biodegradability and non-toxicity (Nazarzadeh Zare et  al., 2019). Therefore, it gained favor in the food industry to be used as a food thickener, stabilizer, emulsifier and gelling agent. With the growing demand for polysaccharides in recent years, it becomes imperative to explore alternative sources for polysaccharides rather than relying solely on natural polysaccharides derived from animals, plants or algae. This diversification is essential to ensure food security and supply. Therefore, microbial polysaccharides can serve as an alternative source

Biopolymer Composites

27

of polysaccharides since they are more consistent in physical and chemical characteristics compared to natural polysaccharides, and they can be modifed to meet targeted needs. Extensive studies have been done to fabricate biopolymer blends or bionanocomposites using microbial polysaccharides through cross-linking with proteins or other biopolymers and even incorporation with fllers to enhance the performance of the material.

1.6

CHEMICAL SYNTHESIS

Synthetic polymers can be synthesized either via chemical or fermentation techniques from renewable resources or fossil fuel. These groups of polymers are classifed under synthetics and sub-classifed into renewable resources and petroleum-based resources. Among the common biobased polymers synthesized from renewable resources are PLA, polyhydroxybutyrate, bio-polyethylene and bio-polypropylene, whereas biodegradable synthetic polymers that are synthesized from petroleum-based resources include polycaprolactone, polybutyrate adipate terephthalate, polyvinyl alcohol and polybutylene succinate. Synthetic or modifed polymers from renewable resources have played a key role in biobased plastic production. Based on their biodegradability, this group of polymers can be further divided into two groups: biodegradable and non-biodegradable polymers. Two common examples of biodegradable polymers are polylactic acid and polyhydroxybutyrate, while two examples of non-biodegradable polymers are bio-polyethylene and bio-polypropylene. On the other hand, certain polymers that are chemically synthesized from petroleum are biodegradable. Some noted examples include polycaprolactone PCL, polybutyrate adipate terephthalate, polyvinyl alcohol and polybutylene succinate. The bonus for this group of polymers is their biodegradability despite exhibiting similar properties to non-biodegradable petroleum-based polymers (Babu et al., 2013; RameshKumar et al., 2020).

1.6.1

RENEWABLE RESOURCES

Synthetic polymers derived from renewable resources are usually acquired either through chemical modifcation of natural biopolymers such as starch, cellulose or chitin or a two-step process from lignin, starch, plant oil and cellulose. This subsection covers a few promising biobased synthetic polymers that have been use in diverse applications from packaging to pharmaceutical and automobiles (Table 1.8). Among these polymers are PLA, polyhydroxybutyrate, polybutylene succinate, biopolyethylene and bio-polypropylene. PLA received great attention in industry and is currently one of the leading biobased polymers used conventionally. PLA is produced from lactic acid, which is naturally present in organic acid via fermentation of sugars from sugar cane, sugar beet or starch. PLA can be synthesized using various established polymerization techniques (Ekiert et  al., 2015). These techniques include polycondensation, ring opening polymerization and direct methods such as azeotopic dehydration and enzymatic polymerization. At present, direct polymerization and ring opening polymerization are among the most employed techniques. Several advantages can be found in PLA. These include energy saving in production, where it uses about 25–55% less energy compared to petroleum-based polymer production (Ekiert et al., 2015).

28

TABLE 1.8 The Source, Physical Features and Applications of Polymers Synthesized from Renewable Resources Polymers

Source

Physical Features

Applications

Slight yellowish hue in its natural form, low glass transition temperature (typically between 111 and 145°F) Stiff and brittle in nature, with low thermal stability and a high degree of crystallinity

Food handling and medical implants, packaging, bottles, biodegradable medical devices, electronic devices

(Aggarwal et al., 2020; Ekiert et al., 2015)

Disposable razors, utensils, diapers, containers, sutures, scaffolds, flms, paper laminates, bags, containers, automobiles Agricultural flms, drug encapsulation, bags or boxes for both food and cosmetic packaging Packaging flms, pouches, boxes, containers, nondisposable carpet, piping

(McAdam et al., 2020a)

Packaging flms, bags, containers

(Siracusa & Blanco, 2020)

Derived from corn starch, sugarcane

Polyhydroxybutyrate

Cells of microorganisms, produced industrially through bacterial fermentation

Polybutylene succinate

Derived from polyvinyl acetate from the fermentation of succinic acid

Good oxygen barrier

Bio-polyethylene

Dehydration of bio-ethanol, obtained from glucose (maize, lignocellulose material, wheat, etc.) Butylene dehydration of bio-isobutanol obtained from glucose (maize, lignocellulose material, wheat, etc.)

Not biodegradable, fexible, durable, printable, transparent, heat resistant, glossy Not biodegradable, fexible, durable, printable, transparent, heat resistant, glossy

Bio-polypropylene

(Puchalski et al., 2018)

(Siracusa & Blanco, 2020)

Biopolymers and Biopolymer Blends

Polylactic acid

Ref.

Biopolymer Composites

29

PLA is biodegradable when it is exposed to the active biological environment. It is compostable and recyclable in the industry, and it is biocompatible. PLA polymers range from amorphous glassy to semi-crystalline to high crystalline, giving it better thermal stability than other biobased polymers such as poly (hydroxyl alkanoate) and poly (ethylene glycol) (PEG) (Arrieta et al., 2017a). These properties made it a promising candidate for food handling and medical implants, packaging, bottles, biodegradable medical devices and electronic devices. Polyhydroxybutyrate is the by-product of microbial secondary metabolism from microorganisms found in renewable resources (e.g. food waste). This occurs when a microorganism encounters limited nutrients and an unfavorable environment, as shown in Figure 1.9. The most common synthesis approach is through bacterial fermentation, which highly depends on a central carbon metabolite from acetyl-CoA via enzymatic activities (Surendran et al., 2020). In terms of its properties, PHB is stiff and brittle, low in thermal stability yet high in degree of crystallinity. Most PHB exhibits similar properties to petroleum-based polypropylene. PHB is of interest due to its biodegradability when it is exposed to an active biological environment. Owing to its properties, PHB is used as disposable razors, utensils, diapers, containers, sutures, scaffolds, films, paper laminates, bags, containers and parts of automobiles. PBS is an aliphatic polyester synthesized by polycondensation between succinic acid and 1,4-butanediol (Puchalski et al., 2018). Traditionally, PBS can only be produced from fossil fuel sources. Currently, it can be produced by fermenting succinic acid from renewable feedstock such as sugars, glucose, starch and xylose (Babu et al., 2013). This process has had greater advantages over the chemical process, as it uses renewable feedstock and consumes less energy. Among the famous companies that have developed PBS from renewable feedstock are Mitsubishi Chemical (Japan) and Ajinomoto (Babu et al., 2013). Owing to their high-impact properties, which are similar to those of polypropylene and have the added benefit of being biodegradable, they are applied in a wide range of applications from packaging to agriculture to biomedicine. As mentioned earlier, the starting monomers or base origin of a material are not able to determine the biodegradability of a material; it rather depends on the end-life properties of a material. For instance, bio-polyethylene and biopolypropylene materials are produced from a starting monomer (i.e. glucose) that can be obtained from renewable resources such as maize, lignocellulose material and wheat. However, the chemical modification and process make them chemically

FIGURE 1.9

Schematic illustration of production of PHB from microorganisms.

30

Biopolymers and Biopolymer Blends

identical to petroleum-based polyethylene and polypropylene. Therefore, they possess properties such as fexibility, durability, printability, transparency, heat resistance and non-biodegradability (Siracusa & Blanco, 2020). Bio-PP is produced by dehydration of bio-ethanol from glucose. The process starts off with cleaning, shredding or cutting the sugar cane, maize or wheat to obtain the juice, which contains sucrose and is then fermented to obtain ethanol. After the distillation process, the water is removed. Finally, it goes through subsequent polymerization to obtain the ethylene monomer. A pilot scale test of bio-PP was done by a company known as Braskem, but its process has not been revealed (Siracusa & Blanco, 2020). Meanwhile, the process of bio-PP is still under investigation. Generally, bio-PP is obtained through dehydration of bio-isobutanol from glucose, which is obtained from renewable feedstock and goes through polymerization.

1.6.2 PETROLEUM-BASED RESOURCES Polymers in this group are derived from fossil fuels, such as synthetic aliphatic polyesters, which are obtained from petroleum. In fact, research is still ongoing for some of these polymers to be synthesized from renewable resources to safeguard our fossil fuels. The unique property of the polymers under this group is the ability to biodegrade even though the source is crude oil or fossil-fuel. Polycaprolactone, polybutyrate adipate terephthalate and polyvinyl alcohol are among the common synthetic biodegradable polymers in this group (Table 1.9). TABLE 1.9 The Source, Physical Features and Applications of Polymers Synthesized from Petroleum-Based Resources Polymer

Physical Features

Applications

Polycaprolactone

Glass transition temperature of about −60°C, good water, oil, solvent and chlorine resistance

(Espinoza et al., 2020; Shahverdi et al., 2022)

Polybutyrate adipate terephthalate

Flexible and tough, which makes it ideal for combination with other biodegradable polymers that have high modulus and strength but are very brittle Crystal clear, water solubility

Scaffold, drug delivery, implants, dermal fllers, root canal flling, wound dressings, contraceptive devices, fxation devices and tissue engineering Garbage bags, wrapping flms, disposable products (lunch boxes, dishes, cups, etc.,) courier bags, mulch flm, shape memory composites, drug delivery

Papermaking, textiles, a variety of coatings, thermal conductor, cementitious composites

(Aslam et al., 2018; Settier-Ramírez et al., 2020; Q. Wang et al., 2023)

Polyvinyl alcohol

Ref.

(de Matos Costa et al., 2020; C. Xie et al., 2023)

Biopolymer Composites

31

Similar to the properties of PLA yet different in terms of raw material, polycaprolactone is chemically synthesized by ring-opening polymerization of carprolactone monomer, the raw material for which can be obtained from crude oil (Di Foggia et al., 2010). PCL is an aliphatic polyester that is able to fully biodegrade and is compostable in an industrial composting system. It is partially crystalline and has a low melting point and a glass transition temperature. This unique property allows it to be used for 3D printing, heat molding and shape memory (Espinoza et al., 2020). The general applications of PCL are scaffolding, drug delivery, dermal fllers, root canal flling and many more in biomedical applications. Polybutyrate adipate terephthalate is a semi-aromatic copolyester of adipic acid that can be synthesized from 1,4-butanediol and adipic acid and the polymer of dimethyl terephthalate (DMT), the raw material for which is crude oil (de Matos Costa et  al., 2020). PBAT consists of aromatic fractions that promote excellent physical properties, while its aliphatic chains exhibit degradation properties, particularly attributed to the cleavage of ester linkages and the interaction between water and their carbonyl groups from benzene rings (de Matos Costa et al., 2020; X. Wang et al., 2019). Thus, PBAT is applied and blended with other thermoplastics in disposable products such as garbage bags, disposable cutlery and disposable wrappers. Polyvinyl alcohol is a synthetic polymer prepared by hydrolysis of polyvinyl acetate (Settier-Ramírez et al., 2020). Polyvinyl acetate is made from acetic acid and ethylene. Ethylene is often produced from natural gas or petroleum. PVOH contains a large amount of hydroxyl groups, which are ready to form hydrogen bonding with water, making them hydrophilic and soluble in water (Aslam et al., 2018). Due to this interesting feature, PVOH has benefted a variety of industries, including papermaking, textiles and coatings.

1.7 NATURAL POLYSACCHARIDES The chemistry of natural polysaccharides, including those from plants, animals and microorganisms, are described in this subsection. These polysaccharides include cellulose, pectin, starch, agar, carrageenan and alginate from plant/algae-based resources; chitosan and hyaluronic acid from animal-based resources; and glucan, pullulan, levan, bacterial cellulose and xanthan gum from microorganisms. This subsection frst presents natural polysaccharides derived from plant, followed by animals and microorganisms. Biopolymers produced from natural polysaccharides offer remarkable advantages, such as renewability, biodegradability and biocompatibility. Enhanced performance can be attained by blending and incorporating fllers (Torres et al., 2019). Therefore, each section covers not only the basic chemistry of the biopolymers but also examples of biopolymers used for biopolymer blends and bionanocomposites.

1.7.1

CELLULOSE

Cellulose is abundantly found in the cell wall of plants and consists of lignin, hemicellulose and cellulose. Each single glucose unit pairs with intermolecular hydrogen

32

Biopolymers and Biopolymer Blends

FIGURE 1.10 Schematic structure of plant cellulose microfibrils.

FIGURE 1.11

Chemical structure of cellulose.

bonding to form cellulose microfibrils (Figure 1.10), which contain about 18 cellulose chains (Kubicki et al., 2018). Cellulose is a polysaccharide with a direct chain of 200 or 300 to over 10,000 β (1→4) associated D-glucose units (C6H10O5)n. Cellulose consists of glucose–glucose linkages of 1–4-linked d-glucopyranosyl units arranged in linear chains. The chains form a network of crystalline fibers known as microfibrils, which are approximately 3 nm in diameter (Aggarwal et  al., 2020). The chemical structure of cellulose is shown in Figure 1.11. Basic physical and chemical characteristics of cellulose are density are around 1.5 g/cm 3, melting point within 260–270ºC, insoluble in organic solvents; it reacts with halogens, nitric acid and sulfuric acid, and its backbone of sugar forms hydrogen bonds and holds cellulose microfibrils together. The type

Biopolymer Composites

33

of bonds that build up cellulose are glycosidic bonds (cellulose is able to form hydrogen bonds with water due to the oxygen atoms in the OH group of cellulose). It also forms inter- and intra-molecular hydrogen bonding between itself and other materials consisting of hydroxyl groups (Aggarwal et al., 2020; Hasan et al., 2018; Wu et al., 2018). Occurrence: Plants, bacteria, tunicates (sea squirts), wood, algae, etc. (Kargarzadeh et al., 2017). Biopolymer blends/bionanocomposites: Silver nanoparticles (AgNPs) are used as a bactericidal agent in active food flm packaging to produce flm with good mechanical and water barrier properties (Hasan et  al., 2018). Cellulose–starch hybrid flms enhanced mechanical properties attributed to hydrogen bonding between the amylopectin of starch and cellulose used for packaging (Noorbakhsh-Soltani et al., 2018).

1.7.2

STARCH

In leaves and other green tissues, starch is a product of photosynthesis and is temporarily stored in chloroplasts during the day and subsequently broken down during the night. This process is known as “transitory starch” and serves to provide a continuous supply of carbohydrates and energy in the absence of photosynthesis. Many plants, including crop plants like wheat and potatoes, generate starch in their seeds and storage organs such as in their grains and tubers. It is used for germination and sprouting purposes for plants. Starch is a polysaccharide that contains a chain of glucose molecules which are bound together. Depending on the plant, starch is made up of between 20–25% amylose and 75–80% amylopectin (Araújo et al., 2020). It is a semi-crystalline polymer consisting of (1–4) linked α-D glucopyranosyl units (Abdul Khalil et al., 2017). The chemical structure of both amylose and amylopectin is illustrated in Figure 1.12. The basic chemical formula of a starch molecule is (C6H10O5)n. There are two types of polysaccharides found in starch: 1 Amylose: a linear chain of glucose 2 Amylopectin: a highly branched chain of glucose Basic physical and chemical characteristics of starch are insolubility in cold water, alcohol and other organic solvents; a boiling point of approximately 100°C, similar to cellulose; and the glycosidic bonds are a special type of covalent bond that link from 500 to several hundred thousand glucose monomers together. The linear to branched glucosyl units are composed of hydroxyl groups, which are able to form inter- and intra-molecular hydrogen bonds with other materials that contain hydroxyl groups. The ratio of amylase and amylopectin plays an important role in mechanical properties, as amylase promotes better flm properties than amylopectin due to its linear component and consists of water residue, which also contributes to the mechanical properties while reducing glass transition temperature at the same time (Abdul Khalil et al., 2017; Wang et al., 2015).

34

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FIGURE 1.12

Chemical structure of starch.

Occurrence: The roots of the cassava plant; the tuber of the potato; the stem pith of sago; and the seeds of corn, wheat and rice (Araújo et al., 2020). Biopolymer blends/bionanocomposites: Starch is hydrophilic and consists of many OH groups. Therefore, enhancement is needed to expand its application. From biopolymers to biocomposites, enhancement of film properties can be done through incorporation of inorganic fillers such as calcium carbonate, kaolinite, silicate, clay and titanium dioxide (TiO2) (Abdul Khalil et al., 2017).

1.7.3

pecTin

Pectin is a structural heteropolysaccharide, which has been used extensively for many years in the food and beverage industry as a thickening agent, a gelling agent and a colloidal stabilizer. It is able to form gel in water under suitable conditions (Naqash et al., 2017). The distinct chain of pectin consists of esterified D-galacturonic acid in an α-1,4linked d-chain, which is covalently linked through α-1,2 bonds to methoxy groups in the natural product (Figure 1.13). These uronic acids have carboxyl groups. Pectins are divided into two categories based on their degree of methylation,

Biopolymer Composites

FIGURE 1.13

35

Chemical structure of pectin.

partially or fully methyl esterifed, including low-methoxyl pectins (degree of esterifcation (DE) < 50%) and high-methoxyl pectins (DE > 50%) (Abdul Khalil et al., 2017). The basic physical and chemical characteristics of pectin are that protopectin is insoluble in water. However, when the fruit ripens, it is converted to water soluble. The monovalent cation salts of pectinic and pectic acids are usually soluble in water. The ability to form gels and gel formation are caused by hydrogen bonding between free carboxyl groups on the pectin molecules and also between the hydroxyl groups of neighboring molecules. Gel strengths increase upon increasing calcium ion concentration. The melting point of pectin is 140 to 180°C (decompose), and the gelling temperature is in the range of 40 to 100°C. The pH of low-methoxyl pectin ranges from 3 to 3.5, while the pH of high-methoxyl pectin ranges from 2.8–3.6 (Naqash et al., 2017). Occurrence: Pectin can be derived from protopectin found in the primary cell walls of terrestrial plants and cell walls of higher plants such as citrus peels or apple skin (Hassan et al., 2019). Biopolymer blends/bionanocomposites: Pectin is generally hydrophilic, which means it is sensitive towards moisture and relative humidity. Hence, blending with other biopolymers such as cellulose, starch or PLA, or incorporation with fllers such as clay and nanocellulose is usually done to solve the problem. Studies have shown that clay improved the tensile strength of a flm compared to pure pectin flm. Specifcally, periodate-oxidized pectin has been combined with chitosan and gelatin to form different structures in biomedical applications (Abdul Khalil et al., 2017).

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1.7.4 AGAR Agar is a water-soluble long-chain algae-based polysaccharide mainly found in red algae and is widely used in the manufacture of gelatin. It also has added advantages to decrease blood sugar concentration and induce an anti-aggregation effect on red blood cells and displays antioxidant, antitumor and antiviral activities (Shahidi & Rahman, 2018). The chemical formula of agar is C14H24O9. It is a complex mixture of polysaccharides composed of two major fractions, agarose and agaropectin. Agarose is the gelling fraction that forms a neutral linear molecule essentially free of sulfates, the chains of which consist of repeating alternate units of β-1,3-linked-D-galactose and α-1,4-linked 3,6 anhydro-L-galactose units. Agaropectin, the non-gelling fraction, is a sulfated polysaccharide composed of agarose and varying percentages of ester sulfate, D-glucuronic acid and small amounts of pyruvic acid (Abdul Khalil et al., 2017). The basic physical and chemical characteristics of agar are that it is insoluble in cold water, but it swells considerably, absorbing as much as 20 times its own weight of water. It dissolves readily in boiling water and sets to a frm gel at concentrations as low as 0.50%. Powdered dry agar-agar is soluble in water and other solvents at temperatures between 95 and 100ºC. Thermo-reversible gel and agar-agar solution in hot water form a characteristic gel after setting, with a melting point between 85 and 95°C and a gelling point between 32 and 45°C. The viscosity of an agar solution at temperatures above its gelling point is relatively constant at pH ranges from 4.5 to 9.0 and is not greatly affected by ionic strength in pH ranges from 6.0 to 8.0. However, viscosity at constant temperature increases with time once gelation begins. Exposure to high temperature and lower pH may result in lower gel strength Thus, exposing agar-agar solutions to high temperatures above 95ºC and pH lower than 6.0 for prolonged periods of time should be avoided (Cotas et al., 2020; Zhang et al., 2017). Occurrence: From red seaweeds such as Gracilaria sp., Gracilaria cornea, Gracilaria dominguensis, Gigartina sp. and Gelidium sp (Cotas et  al., 2020). Biopolymer blends/bionanocomposites: A wide range of studies on biopolymer blends and composites have been done, such as agar/starch blend, agar/ chitosan blend, soy protein, agar blend, agar/nanocellulose and agar/silver nanoparticles for the use of drug carrier and wound dressing (Abdul Khalil et al., 2017). Agar-graft-PVP and κ-carrageenan (seaweed polysaccharides)graft-PVP blends in an aqueous medium of pH 7 are capable of forming hydrogels (Prasad et al., 2006). This study demonstrates that graft blends based on seaweed polysaccharides, as described, hold promise for various applications in biomedicine, including tissue engineering, agriculture for water retention, microbiology and pharmaceuticals as hydrogel dressings. These hydrogels can also serve as a sustainable alternative to animal-derived collagen-based materials. An additional study involving agar aerogel incorporating a small-sized zeolitic imidazolate framework and carbon nitride

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for solar water purifcation introduces an innovative, cost-effective and convenient solar-triggered solution for sustainable water decontamination using a MOF-based aerogel (Zhang et al., 2017).

1.7.5

CARRAGEENAN

Carrageenan is another type of polysaccharide obtained from red seaweed of the class Rhodophyceae. It appears to be a promising candidate in tissue engineering and regenerative medicine in recent years. The molecular formulae of carrageenan is C23H23FN4O7Zn. There are three main isomers composed of iota (ι), kappa (κ) and lambda (λ) ι-carrageenans. The difference between these three is the number and position of ester sulfate groups on the repeating galactose units per disaccharide unit. The characteristics of carrageenan are infuenced by the sulphate ester group of 3, 6 anhydro-galactose content. The chemical structure of carrageenan is shown in Figure 1.14. It is an anionic sulfated linear polysaccharide formed by a straight chain backbone structure of alternating 1, 3-linked-β-D galactopyranose and 1–4 linked α-D-galactopyranose units. These units occur as the 2- and 4-sulphate or unsulfated, while the 4-linked units occur as the 2-sulphate, 2, 6-disulphate, 3, 6 anhydrid and 3, 6 anhydrid 2-sulphate (Abdul Khalil et al., 2017). The basic physical and chemical characteristics of carrageenan are that carrageenan is a water-soluble linear sulphated polysaccharide. All carrageenan types are soluble in hot water at temperatures above its gel melting temperature. The normal amplitude of solubility temperature is between 40 and 70°C, depending on the solution concentration and the presence of cations. In cold water, only lambda-carrageenan and the sodium salts of kappa and iota carrageenan are soluble. Potassium and calcium salts from kappa- and iota-type carrageenan are not soluble in cold water but will swell as a function of concentration and type of cations present as well as water temperature and condition of dispersion. Hot aqueous solutions of kappa and iota carrageenan have the ability to form thermo-reversible gels upon cooling.

FIGURE 1.14 Chemical structure of kappa, iota and lambda carrageenan.

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Biopolymers and Biopolymer Blends

This phenomenon occurs due to the formation of a double helix structure by the carrageenan polymers. The gel is thermo-reversible. The carrageenan concentration is generally 1.5% by weight of the water solution. Commercial carrageenans are generally available in viscosities ranging from about 5 to 800 cps when measured in 1.5% solutions at 75°C. Carrageenan solutions are quite stable in neutral or alkaline pH (Aggarwal et al., 2020; Seo & Yoo, 2021). Occurrence: Red algae from Eucheuma (kappaphycus), Chondrus, Gigartina and Hypnea species (Aggarwal et al., 2020). Biopolymer blends/bionanocomposites: Carrageenan has been incorporated with fllers and blended with other biopolymers to achieve better physical and mechanical properties. Kappa carrageenan shows a synergy effect with locust beam gum (LBG) in aqueous gel systems. Carrageenan is different from other hydrocolloids, as it is able to interact with milk proteins. Interaction occurs due to the strong electrostatic interaction between the negatively charged ester sulfate groups in the carrageenan molecule with the strong positive charges of the milk casein micella (Seo & Yoo, 2021). Another form of interaction is through links established among ester sulfate groups of carrageenan with carboxylic residues of amino acids that make up the protein. Carrageenan has also been incorporated with mica nanoclays and blended with a pectin matrix to achieve 10% higher mechanical strength compared to the neat flm (Abdul Khalil et al., 2017).

1.7.6

ALGINATE

Alginate is another prominent algae-based polysaccharide. It is obtained from brown algae, whose occurrence is mentioned subsequently. The biocompatibility and nontoxicity of alginates make them available for extensive applications, particularly in biomedical felds. The chemical structure of alginate is shown in Figure 1.15. Alginate is mainly composed of uronic acids of M and G blocks, where M blocks refer to 1,4-β-D-mannuronic acid and G blocks refer to 1,4-α-L-guluronic acid, with

FIGURE 1.15 Chemical structure of alginate.

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a homogeneous (poly-G, poly-M) or heterogeneous (GM) block composition, which was proven by partial acid hydrolysis (Wang et  al., 2019). Each species of brown seaweed differs in composition and sequences. Such differences usually occur in the ratio of mannuronic and guluronic acid blocks (Abdul Khalil et al., 2017). The basic physical and chemical characteristics of alginate are its ability to form a uniform and transparent gel when alginic acid interacts with present metal ions such as calcium, sodium or magnesium. It is insoluble in water and organic solvents and thermo-irreversible in room temperature. Alginates have the ability to absorb 200–300 times of their own weight in water to form a viscous gum. The viscosity of alginate solutions increases upon decreasing pH and reaches a maximum around pH 3 to 3.5 due to the protonated alginate backbone that forms hydrogen bonds (Usman et al., 2017; Wang et al., 2019). Occurrence: Extracted from brown algae (Phaeophyceae), including Laminaria japonica, Laminaria digitata, Laminaria hyperborea, Macrocystis pyrifera and Ascophyllum nodosum (Wang et al., 2019). Biopolymer blends/bionanocomposites: Many studies have been done to improve the weak mechanical and water barrier properties of the alginate matrix. Some studies have incorporated other polysaccharides such as PLA, chitosan and hyaluronic acid (HA), as well as soy protein, while others incorporated minerals as fller in the alginate matrix (Kosik-Kozioł et al., 2017; Wang et al., 2019; Wongkanya et al., 2017). Such incorporation includes cellulose nanofbrils and calcium chloride. This process is also applied in pharmaceuticals as a coating flm on drug tablets to control the release of drug (Volić et al., 2018).

1.7.7

CHITOSAN

Chitosan is a partially deacetylated product of chitin. Chitosan is widely used in the pharmaceutical feld, as it contains positive charges that can interact with the negative part of cell membrane. Unlike other polysaccharides that usually contain carbon, hydrogen and oxygen, chitin and chitosan consist of nitrogen (6.89%) (da Silva Alves et al., 2021). The formula of chitosan is C6H11O4N, and it is a copolymer made up of one amine and two free hydroxyl groups for each monomer. It comprises of β-(1 → 4)-2-acetamido-D-glucose and β-(1 → 4)-2-amino-D-glucose units (da Silva Alves et al., 2021). Chitosan contains two main sugars, glucosamine and N-acetylglucosamine; the proportion for each sugar usually relies on the alkaline treatment (Jiménez-Ocampo et al., 2019). The basic physical and chemical characteristics are that it is an off-white powder with molecular weight 300–1000 kDa depending on the source of chitin. The degree of acetylation (DA) is 15–25%. Viscosity is 86.4 Pa s (at 20°C, 30 rpm). It has no melting point, as it decomposes upon heating, and chitin and chitosan also contain nitrogen (6.89%), which is the reason for their being of commercial interest. Chitosan is the only positively charged, naturally occurring polysaccharide. Chitosan molecules have both amino and hydroxyl groups. Stable covalent bonds can be formed via various processes including etherifcation, esterifcation and reductive amination.

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Biopolymers and Biopolymer Blends

Chitosan has antibacterial activity and antifungal, mucoadhesive, analgesic and hemostatic properties (Jiménez-Ocampo et al., 2019; Qiao et al., 2017). Occurrence: Insects, crustaceans, squid, centric diatoms and fungi (Hassan et al., 2019; Qiao et al., 2017). Biopolymer blends/bionanocomposites: Both chitin and chitosan possess reactive hydroxyl and amino groups, but chitosan is usually less crystalline than chitin, which makes chitosan more accessible to reagents and other biopolymers. Several examples of the usage of chitosan for emerging applications include blending it with gelatin and 3-phenylacetic acid for food packaging applications (Liu et al., 2021). Additionally, a chitosan/gelatin/ PVA blend was formulated into a hydrogel for wound dressings (Fan et al., 2016), and chitosan/starch/silver nanoparticles were employed to create antimicrobial papers (Jung et al., 2018).

1.7.8

HYALURONIC ACID

Hyaluronic acid is also known as hyaluronan. It receives tremendous attention, particularly in the feld of cosmetics and pharmaceuticals. It is an essential component found in the extracellular and pericellular matrixes and the inner cells. It is an unbranched biopolymer, which belongs to heteropolysaccharides. The formula of hyaluronic acid is C14H21NO11 n., and the chemical structure is illustrated in Figure 1.16. HA has a linear chain consisting of repeating disaccharide units linked by ß-1,4-glycosidic bonds. Each disaccharide contains N-acetyl-D-glucosamine and D-glucuronic acid connected by ß 1,3-glycosidic bonds. The basic physical and chemical characteristics are that HA can reach a very high molecular weight, up to 20,000 kDa. HA possesses fundamental physical and chemical characteristics, such as being able to attain an exceptionally high molecular weight, reaching up to 20,000 kDa. Furthermore, HA is notably sensitive to pH,

FIGURE 1.16

Chemical structure of HA.

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41

undergoing hydrolysis in both acidic and alkaline environments, and depolymerizes at pH levels exceeding 11 or dropping below 4. HA solution is a non-Newtonian liquid with shear-thinning and viscous behavior. HA has hydrophilic sites such as carboxylic acid (-COOH) or hydroxyl (-OH) groups that readily link with other hydrophilic substrates (Fallacara et al., 2018). Occurrence: Rooster combs, human umbilical cords, human joint synovial fuid, vitreous humor, human dermis and epidermis, vertebrate tissues and the skin of animals (Fallacara et al., 2018). Biopolymer blends/bionanocomposites: HA degrades easily and exhibits weak biomedical properties. Hence, there is always a need to improve its performance for end use. Its biocompatibility and free hydroxyl groups allow it to crosslink with other biopolymers such as gelatin and chitosan (Alemdar, 2016). There is also evidence of hyaluronic acid blended with poly (L-lactic acid) for biomedical application (Wang et  al., 2017). More recently, chitosan is blended with hyaluronic acid as hydrogel for injectable tissue engineering as well (Lee et al., 2020).

1.7.9 YEAST GLUCAN Glucan is abundant in the cell walls of fungi. β-glucans are carbohydrates (sugars) that are found in the cell walls of bacteria, fungi, yeasts, algae, lichens and plants such as oats and barley. They are taken as herbal medicines; to prevent and treat cancer, human immunodefciency virus (HIV) and diabetes; to lower cholesterol; and to increase immune system function. Therefore, glucan is particularly popular in the food and pharmaceutical industries (Yuan et al., 2020). Recently, the application of glucans has been extended to water treatment owing to their effectiveness in heavy metal adsorption (Jiang et al., 2019). Yeast glucan belongs to the class of β-glucan, and its structure includes two distinct macromolecular components composed of consecutively (1→3)-linked β-D-glucopyranosyl residues, with small numbers of (1→6)-linked branches and a minor component with consecutive (1→6)-linkages and (1→3)-branches (Rahar et al., 2011). Yeast glucan exhibits fundamental physical and chemical characteristics, including a high degree of polymerization (DP > 100). β-glucan is composed of multiple OH groups and β-(1→3) linkages, and its high DP renders it completely insoluble in water. The solubility of yeast glucan increases as the DP decreases. The ratio of soluble to insoluble fractions of β-glucan is signifcantly infuenced by the extraction conditions (Rahar et al., 2011; Yuan et al., 2020). Occurrence:Rhynchelytrum repens, Lentinus edodes, Grifola frondosa, Tremella mesenterica, Tremella aurantia, Zea may, Agaricus blazei, Phellinus baummi, Saccharomyces cerevisae (yeast) and Agaricus blazei murell (mushroom) (Rahar et al., 2011; Yuan et al., 2020). Biopolymer blends/bionanocomposites: The large number of hydroxyl groups in β-glucan allow interaction with other biopolymers. Such evidence is found in brown wheat four/β-glucan blended with xanthan and guar gum to enhance

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the food texture, which is absolutely benefcial for frozen dough (Ahmed & Thomas, 2018). Apart from that, the biocompatibility of glucan with multiple OH groups allows them to be easily crosslinked with chitosan and the formation of hydrogels for heavy metal adsorption (Jiang et al., 2019).

1.7.10

PULLULAN

Pullulan is a non-ionic polysaccharide derived from fungus (Figure 1.17). Due to its unique properties such as high solubility and low viscosity, and because it is easily modifed and has stability in a wide range of pH values, pullulan has been used in many applications, including as a food additive, blood plasma substitution, focculant, and adhesive and in flm fabrication (Singh & Kaur, 2019). It is now becoming more competitive with natural gums made from marine algae and other plants in terms of cost effectiveness. The chemical formula of pullulan is (C6H10O5)n. Pullulan is a linear, non-ionic polysaccharide with a chemical structure composed of maltotriose units: α-(1 → 6)-linked (1 → 4)-α-d-triglucosides (Priyadarshi et al., 2021). The basic physical and chemical characteristics of pullulan are that it is a white powder, odorless and tasteless. It is highly soluble in water and diluted alkali due to a low degree of hydrogen bonds in its crystal form. It has high fexibility of the chain and forms a viscous solution in hot and cold water but does not form a gel. It is less viscous compared to other hydrocolloids, with high stability to sodium chloride and a wide range of pH values from 2 to 11. It is heat resistant and decomposes at high temperatures from 250 to 280°C. It is insoluble in organic solvents, except in dimethylformanide and dimethyl sulfoxide, and the α-1,6-glucosidic linkages in pullulan give structural fexibility (Wang et al., 2019; Priyadarshi et al., 2021). Occurrence: Fermented from black yeast Aureobasidium pullulans and other species of microorganisms, Tremella mesenterica, Teloschistes favicans, Cryphonectria parasitica, Cytariaharioti and different carbon sources from Asian palm kernel, jackfruit seed and rice hull (Priyadarshi et  al., 2021; Singh & Kaur, 2019).

FIGURE 1.17

Chemical structure of pullulan.

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Biopolymer blends/bionanocomposites: Due to its unique α-1,6-glucosidic linkages, pullulan is fexible enough to form intra- and inter-molecular bonding with pectin, which eventually enhances water resistance properties (Priyadarshi et  al., 2021). This is useful, especially in food packaging applications. Another instance is found in gelatin and pullulan blends incorporated with nanofbers. This composite showed enhanced mechanical properties due to the increase of intermolecular hydrogen bonds and the decrease of intramolecular hydrogen bonds. Because of that, this composite was suggested to be used in tissue engineering (Wang et  al., 2019). The modifcation of pullulan, performed via chemical reactions, blending or incorporation of fllers is usually employed to enhance its uses in as many areas as possible.

1.7.11

DEXTRAN

Dextran is another exopolysaccharide obtained from bacteria in a sucrose-rich media (Ghimici & Nichifor, 2018). It is highly demanded in biomedical, pharmaceutical and tissue engineering applications. This is because it is biocompatible, non-toxic and non-immunogenic (Zheng et al., 2019). Conventionally, it has been used to treat hypovolemia resulting from surgery and other types of bleeding. The formula of dextran is H(C6H10O5)xOH, and the chemical structure of dextran is shown in Figure 1.18. It is a hydrophilic polysaccharide composed of α(1,6)-linked glucopyranose units

FIGURE 1.18 Chemical structure of dextran.

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with α-1 → 2, α-1 → 3, and α-1 → 4 linked side chains attached to the C-3 position of the backbone (Rzayev et al., 2017; Ghimici & Nichifor, 2018). Occurrence: Produced by bacteria strains from Leuconostoc, Lactobacillus and Streptococcus (Ghimici & Nichifor, 2018; Hassan et al., 2019). Biopolymer blends/bionanocomposites: Dextran can be easily modifed to form a 3D network structure with some of the promising functional groups including tyramine, ethylamine vinyl sulphones, thiols and acrylates (Ghimici & Nichifor, 2018). Dextran is used in biomedical applications owing to its hydroxyl groups, which can be oxidized into aldehydes and function as a crosslinking agent to crosslink with other amino groupcontaining polymers. One instance of this is the fabrication of a poly (vinyl alcohol)/dextranaldehyde composite hydrogel for wound dressing application (Zheng et al., 2019). It is also a versatile biopolymer to fabricate electrospun nanofber membranes. These can be found in colloidal nanofber composites of dextran and folic acid for electro-active platforms (Rzayev et  al., 2017). Increasing the fraction of dextran in nanofber colloids is believed to improve the transmission of folic acid due to the hydrogen bonding linkages.

1.7.12 BACTERIAL CELLULOSE Bacterial cellulose (BC) is produced via bacteria fermentation in a static culture. Recently, a signifcant expansion of applications in antibacterial packaging applications in the food and packaging industries and drug delivery systems has been noticeable (Rydz et  al., 2018; Treesuppharat et  al., 2017). The BC chemical structure is illustrated in Figure 1.19. BC is made up of (1 → 4)-D-anhydroglucopyranose chains bonded through β-glycosidic linkages. The material’s geometry is determined by the intra-molecular and inter-molecular hydrogen-bonding network and hydrophobic and van der Waals interactions, forming parallel chains (Rangaswamy et al., 2015). The basic physical and chemical characteristics of BC are its high crystallinity (84–89%) and high polymerization degree. It has a higher surface area than the

FIGURE 1.19 Chemical structure of bacterial cellulose.

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cellulose obtained from plant sources (high aspect ratio of fbers with diameter 20–100 nm), high fexibility (Young’s modulus of 15–18 GPa) and high waterholding capacity (over 100 times its own weight) with the specifc surface area (37 m2/g) (Rangaswamy et al., 2015) (Treesuppharat et al., 2017). Occurrence: Bacterial cellulose is produced extracellularly by gram-negative bacterial cultures of Gluconacetobacter, Acetobacter, Agrobacterium, Achromobacter, Aerobacter, Sarcina, Azobacter, Rhizobium, Pseudomonas, Salmonella and Alcaligenes. Among the bacteria, Komagataeibacter xylinus is the most effcient in producing bacterial cellulose (Rangaswamy et al., 2015). Biopolymer blends/bionanocomposites: In biomedical applications, bacterial cellulose nanocrystals/regenerated chitin fbers (BCNC/RC) for suture biomaterial, 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-oxidized BC with Ag nanoparticles, have been developed for wound dressing application (Wu et  al., 2018). Another biomedical application for drug delivery was achieved by bacterial cellulose/gelatin hydrogel composites crosslinked with glutaraldehyde. The composite showed excellent dimensional stability caused by the intra- and inter-molecular hydrogen bonding interaction between bacterial cellulose and gelatin (Treesuppharat et  al., 2017). For packaging applications, it was observed that BC blended with PCL showed better mechanical properties and biodegradability than neat PCL due to the contribution of high tensile properties of BC due to homogenous distribution of PCL throughout the BC network (Rydz et al., 2018).

1.7.13

XANTHAN GUM

Xanthan gum is an anionic heteropolysaccharide produced through the fermentation of simple sugar by a specifc kind of bacteria. It was frst discovered in the 1960s and then commercialized in the 1970s. It is a recognized stabilizer, additive and thickening agent used for toothpaste, cream and lotions, as it is known for its biocompatibility, biodegradability and water solubility (Kumar et al., 2017). The chemical formula of xanthan gum is C35H49O29, and the chemical structure of xanthan gum shows a long-chain polysaccharide, which consists of -glucose, d-mannose and d-glucuronic acid as building blocks with a high number of trisaccharide side chains on every glucose (Figure 1.20). The beta-D-glucoses are linked (1 → 4) to form a backbone similar to cellulose (Sworn, 2021). The basic physical and chemical characteristics are that xanthan gum easily dissolves at room temperature and is soluble in cold water. It has a high molecular weight (on the order of 1000 kDa) and shows high pseudoplastic fow. It has high viscosity and decreases with shear force applied. The viscosity remains the same from 0 to 100°C and from a pH of 1 to 13. It is highly enzyme resistant and has synergistic interaction with galactomannans (e.g. guar gum, konjac and locust bean gum). It is stable in a wide range of pH, temperature and organic solvents. It consists of a large number of hydroxyl groups that are able to form hydrogen bonding with other biopolymers (Kumar et al., 2017; Sworn, 2021).

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FIGURE 1.20 Chemical structure of xanthan gum.

Occurrence: Produced extracellularly by a pure-culture fermentation process of carbohydrate by the bacterium known as Xanthomonas campestris (Aggarwal et al., 2020). Biopolymer blends/bionanocomposites: Xanthan gum can achieve better physical performances when it is blended with other biopolymers and incorporated with fllers such as alginate, cellulose nanocrystals and halloysite nanotubes, mainly due to the formation of electrostatic attraction and hydrogen bonding that promote uniform dispersion and result in enhanced mechanical and thermal properties (Kumar et  al., 2017). A crosslinked hydrogel nanocomposite of xanthan gum for adsorption of crystal violet dye in water purifcation applications showed the effectiveness of dye adsorption due to the presence of electrostatic and hydrogen bonding between the hydroxyl group of xanthan gum and the polar nitrogen atom within the crystal violet dye (Mittal et al., 2021).

1.8

NATURAL PROTEINS

The chemistry of natural proteins, including those mainly from plants and animals, are detailed in this section, starting with natural plant proteins, which include soy protein, zein protein and gluten protein, and followed by animal proteins, including collagen, gelatin and keratin. Similar to the sequence of description in the section on natural polysaccharides, the chemical structure, chemical characteristics and some notable examples of natural proteins used in biopolymer blends or bionanocomposites are detailed accordingly.

Biopolymer Composites

1.8.1

47

SOY PROTEIN

Soy protein can be found in soybeans. One of the important groups of minor compounds present in soybean that has received tremendous attention is a class of phytoestrogen called isofavones. Isofavone compounds were considered non-nutrients, as they did not provide energy and did not even function as vitamins (Dan Ramdath et al., 2017; Yu et al., 2016). However, they play a key role in preventing numerous diseases, so they are often incorporated in health-promoting substances. Daidzein and genistein are the most common isofavones, whose characteristic chemical structure (the B-ring is linked to the C3 position of the C-ring instead of the C2 position) resembles the structure of estrogens, in particular 17-β estradiol (Yu et  al., 2016) (Figure 1.21). The basic physical and chemical characteristics of soy protein are that the molecular weight of soy protein ranges from 300,000 to 600,000 kDa. The polypeptide chains of soy protein polymers are associated with and entangled in a complicated three-dimensional structure by disulfde and hydrogen bonds. Soy protein is abundant, with a great amount of polar functional groups such as hydroxyl, amino and carboxylic groups (Dan Ramdath et al., 2017; Han et al., 2017). Occurrence: Soybeans that have been dehulled and defatted (Aggarwal et al., 2020; Hassan et al., 2019). Biopolymer blends/bionanocomposites: Soy protein has certain drawbacks such as low water resistance and mechanical properties that limit its application. Therefore, soy protein needs to be modifed to improve mechanical properties, water resistance and productive life to facilitate its application (Tian et al., 2018). Recently, soy protein resins have been used to fabricate green composites for many applications such as hydrogels, adhesives, plastics, flms, coatings and emulsifers (Tian et al., 2018). In another instance, water resistance and thermal abilities could improve due to the addition of graphene. This improved performance may be due to the hydrogen bonds and π–π interactions between graphene and the soy protein isolate (SPI) matrix, which gives SPI–graphene flms wide potential application in drug delivery, packaging and the food industry (Han et al., 2017).

FIGURE 1.21

Chemical structure of isofavones and estradiol.

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FIGURE 1.22

1.8.2

Chemical structure of zein.

ZEIN

Zein is a plant protein isolated from corn. It has been found exclusively in corn endosperm cells (Figure 1.22) (Shukla & Cheryan, 2001). Originally, the primary production of zein came from the by-products of starch and oil during the wet milling process, and zein protein was usually incorporated into animal feed. Zein was considered a waste protein and did not receive much attention until recent years. Several uses were found for zein before petroleum, including the production of fbers, adhesives, buttons, binders and coatings. There is currently a growing interest in zein as a polymeric material, in part due to the perceived negative impact of plastic on solidwaste disposal. Moreover, zein offers many advantages as a raw material to produce plastics, coatings and flms (Lorenzo et al., 2018). Zein is a combination of different peptide chains linked by disulfde bonds. These peptides can be classifed according to their solubility, charge and molecular size. There are two major fractions of zein, α and β. α-zein is the major protein; it accounts for about 80% of the total prolamin present in corn and is soluble in 60–95% aqueous ethanol. Besides that, α-Zein contains less histidine, arginine, proline and methionine than β-zein. On the other hand, β-zein accounts for approximately 10% of the total zein content in corn, is relatively unstable, precipitates and coagulates frequently and has therefore not been a constituent of commercial zein preparations. The molecular weight of zein varies from 22 to 27 kDa. It has an isoelectric pH of 6.228 and is the major storage protein (accounting for 35–65%) in corn. Pure zein is clear, odorless, tasteless, hard, water insoluble and edible. One of the defning characteristics of zein is its insolubility in water. This hydrophobicity is attributed to the presence of a high proportion of nonpolar amino acid residues. The poor water solubility and imbalanced amino acid profle make zein a less than ideal protein for human consumption. Zein is soluble only in aqueous ethanol and proteins containing higher concentrations of amino acids, such as glutamic acid, proline, leucine and alanine, and lower amounts of basic and acidic amino acids. Therefore, it is highly hydrophobic. Occurrence: Maize (corn). Biopolymer blends/bionanocomposites: A series of reports showed that due to its nontoxicity, biodegradability and good biocompatibility, zein is a promising natural polymer for use as a scaffold in drug delivery systems, wound healing and food packaging (Lorenzo et al., 2018). It has been successfully

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blended with other biopolymers such as silk, collagen, chitosan, polycaprolactone and poly(L-lactide) (Elzoghby et al., 2015). Additionally, the ability of zein and its resins to form tough, glossy, hydrophobic grease-proof coatings and their resistance to microbial attack has been of commercial interest (Sharif et al., 2019). Potential applications of zein include uses in fber, adhesives, coatings, ceramics, inks, cosmetics, textiles, chewing gum and biodegradable plastics (Lorenzo et al., 2018).

1.8.3

GLUTEN

Gluten is a Latin word that means “glue”, due to its ability to hold grains like wheat, barley and rye together (Biesiekierski, 2017). Gluten is a mixture of hundreds of distinct proteins within the same family, although it is primarily made up of two different classes of proteins: gliadin, which gives bread the ability to rise during baking, and glutenin, which is responsible for dough’s elasticity (Lorenzo et al., 2018; Patni et al., 2011). Research has been performed to develop techniques for converting wheat gluten into more useful products. Plant protein from wheat shows the advantage for usage as flms and plastics because of its abundant resources, low cost and good biodegradability, making it a promising substitute for petroleum-based plastics. Gluten proteins are termed prolamins. They are proteins attached to starch in the endosperm, which consists of gliadin (the water-soluble component) and glutenin (the water-insoluble component); they bind to each other to form a network that supports dough and allows bread to be light and fuffy (Sharma et al., 2017). Amino acids present in both gliadin and glutenin help the two proteins form hydrogen bonds with each other (Sharma et al., 2017). Gluten is a protein complex which is insoluble in water, although there may be amounts of soluble proteins trapped in the gluten matrix. Despite its insolubility and its hydrophobic nature, gluten absorbs about twice its dry weight in water to form the gluten network. It is sticky, extensible and elastic and has high water absorption capacity and cohesivity (Biesiekierski, 2017; Patni et al., 2011). Occurrence: Wheat, rye, barley, triticale, spelt, einkorn, emmer and kumut (Biesiekierski, 2017). Biopolymer blends/bionanocomposites: Many factors are involved in plasticizer selection, including molecular structures, polarities, required product qualities, properties and costs. Various promising plasticizers that can be utilized in making wheat gluten–based bioplastic are glycerol, xylan, dicarboxylic acid, lactic acid, water and octanoic acid. Various crosslinking agents can also be used to improve the properties of the flm, including aldehyde, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, N-hydroxysuccinimide and silica. For instance, additives like xylan or fllers like silica and hydro ethyl cellulose, lubricators like salicyclic acid and binders like urea and sodium hydroxide can be incorporated into gluten in various ratios to form biodegradable composite flms (Patni et  al., 2011; Sharma et al., 2017). Furthermore, there is evidence of electrospun nanofbers blended with gluten for biomedical applications (Aziz et al., 2019).

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1.8.4

COLLAGEN

Collagen refers to a family of proteins that are the primary structural component of connective tissues, such as skin and cartilage. The substance makes up about a third of all the protein within the mammalian body, more than any other type of protein in the body by mass. There are 29 known types of collagen (Yoon et al., 2020). The confguration of this protein greatly affects its role in tissue architecture. Collagen contains many periodically repeating 3-amino-acid sequences containing Gly. The Gly consists of repeating sequences, including about 1,400 amino acid residues. The most common tripeptide unit of collagen is Gly-Pro-Hyp, and these three residues form one helical turn. There are several collagens, but 80–90% of them belong to types I, II and III, for example, type II collagen with a molecular mass of 1461.64 g mol−1 and a molecular formula of C65H102N18O21 (Figure 1.23). It is often found to group in three molecules and twist to form collagen aggregation about 290 nm long and 1.5 nm in diameter (Vázquez-Portalatĺn et al., 2016). The melting point of collagen is in the range of 30 to 50°C, and the boiling point is between 160 and 190°C. It is poorly soluble in water and has a high content of alpha helix structures linked together with covalent bonds. As a structural protein, collagen has excellent biocompatibility and cell adhesion and can promote cell proliferation and differentiation (Liu et al., 2019). Occurrence: Collagen occurs throughout the body, but especially in the skin, bones and connective tissues (Hassan et al., 2019). Biopolymer blends/bionanocomposites: Owing to the mechanical structure, collagen has high tensile strength and is a nontoxic, easily absorbable, biodegradable and biocompatible material. Therefore, it has been used for many medical applications such as in treatment for tissue infection, drug delivery systems and gene therapy. Collagen matrices or sponges can be used to treat wounds for tissue regrowth and reinforcement. It is biocompatible and readily crosslinks with other biopolymers such as chitosan and HA, for example, hydroxyapatite/collagen composites, chitosan-collagen composites that have been seeded with cells for tissue-engineered heart

FIGURE 1.23

Chemical structure of type II collagen.

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valves, and 3D composites derived from blends of chitosan and collagen incorporating HA as crosslinker for wound healing applications (Bu & Li, 2018; Fu et al., 2017; Sionkowska et al., 2016).

1.8.5 GELATIN Gelatin is an extracellular matrix protein, which allows it to be used in applications such as wound dressings, drug delivery and gene transfection. There are two types of gelatin, Type A and Type B. Type A gelatin is derived by acid hydrolysis of collagen and has 18.5% nitrogen. Type B is derived by alkaline hydrolysis containing 18% nitrogen and no amide groups. Elevated temperatures cause the gelatin to melt and form coils, whereas lower temperatures result in coil to helix transformation (Ninan et al., 2015). Gelatin is a heterogeneous mixture of single- or multi-stranded polypeptides, each with extended left-handed proline helix conformations and containing between 300 and 4,000 amino acids. The basic chemical structure of gelatin consists of functional groups such as NH2 and COOH (Bazmandeh et al., 2020). Based on the process used for its manufacture, gelatin is obtained either as Type A or B. Type A gelatin is obtained by acidic treatment of collagen and has an isoelectric point (pI) between 7.0 and 9.0. Type B gelatin, however, is obtained by alkaline hydrolysis of collagen and has a pI between 4.8 and 5.0 (Ninan et al., 2015). Gelatin is a polyampholyte that gels below 35 to 40°C. The heterogeneous nature of the molecular weight profle of this biopolymer is affected by pH and temperature, which in turn affects the noncovalent interactions and phase behavior of gelatin in solution. Crosslinking and/or hardening or could be as complex as ligand-mediated active targeting at the cellular level and contains many functional groups like NH2 and COOH, which allow gelatin to be modifed using nanoparticles and biomolecules (Bazmandeh et al., 2020; Voron’ko et al., 2016). Occurrence: Gelatin is a denatured form of collagen obtained by acid or alkaline collagen processing (Derkach et al., 2020). Biopolymer blends/bionanocomposites: Gelatin polymer is often used in dressing wounds, where it acts as an adhesive. Scaffolds and flms with gelatin allow for the scaffolds to hold drugs and other nutrients that can be used to supply a wound for healing (Bazmandeh et al., 2020). However, there are some inherent problems of gelatin-based flms, such as poor mechanical and water resistance. These can be overcome by applying recently developed nanocomposite technology. Gelatin-based nanocomposite flms prepared with various types of nanofllers, such as nanoclay, organic fllers and nanometals, have exhibited increased flm properties, along with other novel properties, such as antimicrobial activity, antioxidant activity and UV-screening properties (Derkach et al., 2020). Subsequently, gelatin-based nanocomposite flms blended with certain organic fllers and inorganic fllers such as nanosilver, nanocopper (CuNPs), zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles exhibited strong antimicrobial activity against foodborne pathogenic microorganisms (Ninan et al., 2015).

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1.8.6

KERATIN

Keratin is the major structural fbrous protein to form the hair, wool, feathers, nails and horns of many kinds of animals. Keratin can be classifed into two groups: soft keratin and hard keratin. It may be present in two conformations, α-helix and β-sheet. Compared to other proteins, keratin-based materials have higher stability and are not degraded by enzymes (Shavandi et al., 2017). The chemical structure of keratin is shown in Figure 1.24. A keratin protein is defned by a primary structure based on amino acid chains. These chains vary in number and sequence of amino acids, polarity, charge and size. However, similarities exist in their structure independently of the species of animal or function. Small modifcations in the keratin’s amino acid sequence cause signifcant property modifcation, since these sequences determine the whole molecular structure and the nature of the covalent or ionic bonds. The sulfur-containing amino acids, methionine and cysteine, as shown in Figure 1.24, have an even greater infuence due to their role in establishing intra- or inter-molecular disulfde bonds (Shavandi et al., 2017). Keratin is extremely insoluble in water and organic solvents and has high sulfur content and flament-forming proteins. It has a high concentration of cysteine, 7 to 20% of the total amino acid residues, that form inter- and intra-molecular disulfde bonds. Cysteine-rich proteins are endowed by nature with high mechanical strength owing to the large number of disulfde bonds, and the elastic nature of keratin fber is due to the interplay between α-helix and β-sheet confguration of the protein. The disulfde linkage between cysteine molecules present as intrachain and interchain bonds is responsible for its good stability and lower solubility. Therefore, the dissolution and the extraction of keratin are diffcult compared to other natural polymers, such as collagen or starch (Hassan et al., 2019; Ma et al., 2017). Occurrence: There are two types of keratin, which can be differentiated as α- and β-keratins. The former is widely found in wool, hair, horn, nails, hooves and the outermost layer of the skin, also known as the stratum corneum. β-keratins are the real part of hard avian and reptilian tissues, for example, plumes, hooks, mouths of fying creatures and scales and paws of reptiles (Hassan et al., 2019; Aggarwal et al., 2020).

FIGURE 1.24

Chemical structure of keratin.

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Biopolymer blends/bionanocomposites: Keratin composites that are prepared without any additives suffer from a brittle structure, which limits their applications. Therefore, several studies have tried to overcome this weak structure by reinforcing with additives or blending with natural or synthetic polymers (Singamneni et al., 2019). Improved mechanical properties were generally achieved. In this regard, there has been increasing interest in reinforcing the keratin matrix with naturally derived green compounds. As an example, keratin blends with chitosan have been proposed for wound healing and artifcial skin substitutes (Lin et al., 2017). On the other hand, the interaction between keratin and synthetic polymers has also been widely and deeply studied. The relationship between poly(ethylene oxide) (PEO) and keratin-blended flms was explored in order to develop a keratin-based material with improved structural properties (Ma et  al., 2017). The improved structural properties of keratin/PEO blends enable the development of keratin materials for use as scaffolds for cell growth, wound dressing and drug-delivery membranes.

1.9

SYNTHETIC POLYMERS FROM RENEWABLE RESOURCES

The chemical structure and characteristics of synthetic polymers from renewable resources are detailed in this section. Synthetic polymers from renewable resources covered in this section are PLA, polyhydroxybutyrate, polybutylene succinate, biopolyethylene and bio-polypropylene. In addition, several examples of using these synthetic polymers in biopolymer blends and bionanocomposites are detailed in this section.

1.9.1 POLYLACTIC ACID Polylactic acid is a promising alternative to petroleum-based plastics that have been widely used in packaging, electronic, automotive and biomedical applications. This is because polylactides can break down into nontoxic products such as water and carbon dioxide during degradation under an active biological environment and are biocompatible (Baran & Yildirim Erbil, 2019). Thus, they reduce the amount of plastic waste. The most common way to produce PLA is through fermentation from corn or sugarcane feedstock, which is from 100% renewable resources. The general chemical structure of PLA is shown in Figure 1.25. PLA is made up of lactic acid monomer (2-hydroxypropanoic acid, HO–CH3–CH–COOH), which is the simplest 2-hydroxycarboxylic acid with a chiral carbon atom. It can exist in two stereocomplex crystallization of enantiomeric forms: L-lactic and D-lactic. Utilizing poly (L-lactide) (PLLA) and poly (D-lactide) (PDLA) represents a robust approach for substantially improving material properties, including stability and biocompatibility. This enhancement arises from the potent intermolecular interactions between L-lactyl and D-lactyl units, which have proven a pivotal strategy in advancing applications of PLA (Baran & Yildirim Erbil, 2019). Pure PLA exhibits a melting point of 180°C and a glass transition temperature of 60°C. PLA, on the other hand, is a hydrophobic polymer with poor toughness and

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FIGURE 1.25 Chemical structure of PLA.

low thermal and crystallization capabilities and is prone to degradation via hydrolysis and occasionally microbial attack, unlike polysaccharides, which possess more reactive side chain groups. PLA exhibits tensile strength, fexural strength and specifc gravity in the ranges of approximately 61 to 66 MPa, 48 to 110 MPa and 1.24 g/cm3, respectively. This is due to the presence of hydroxyl and acid functional groups in the lactic acid molecule, enabling esterifcation reactions (Baran & Yildirim Erbil, 2019; Chotiprayon et al., 2020; Coppola et al., 2018). Occurrence: Produced by polycondensation and/or ring-opening polymerization of lactic acid from the natural feedstock such as corn, potatoes or sugar beets (Baran & Yildirim Erbil, 2019). Biopolymer blends/bionanocomposites: PLA can be blended with other fexible polymers, which can act as plasticizers to improve its mechanical strength and reduce its brittleness. PLA is often blended with starch thermoplastic and coir fbers to increase biodegradability and reduce the cost of the resulting blend (Chotiprayon et al., 2020). The use of chitin as a reinforcing fller provides a promising expansion on the applications of PLA composite flm in the biomedical feld (Olaiya et al., 2019). This is due to PLA composite flm showing a signifcant increase of glass transition, primarily due to the increase of intermolecular bonds between PLA, chitin and starch. PLA usually exhibits a poor gas barrier and water vapor barrier. Therefore, it is often incorporated with nanoclay or carbon nanotubes to promote better thermal and mechanical properties compared to neat PLA (Coppola et al., 2018).

1.9.2

POLYHYDROXYBUTYRATE

Polyhydroxybutyrate is a member of polyhydroxyalkanoate family, which has been extensively studied for commercial use in packaging, medical and coating materials. The interesting feature of PHB is that it has properties similar to petroleum-derived polypropylene, yet it is biodegradable under aerobic and anaerobic condition and biocompatible (Mohandas et  al., 2017). PHB, a homopolymer, contains monomer 3-hydroxybutyric acid (HBA) units. It has a methyl functional group (CH3) and an ester linkage group (-COOR). This chain and functional group play an important role in giving the material thermoplastic, hydrophobic, high crystalline, and brittle characteristics (McAdam et al., 2020b).

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PHB is highly crystalline due to its linear chain structure and shorter pendant groups. It has higher gas barrier properties than polyethylene and polypropylene (PP), polyethylene terephthalate (PET) and polyvinylchloride (PVC) is more rigid but less fexible than PP. It is able to degrade to D-3 hydroxybutyric acid in vivo together with low toxicity. It is thermally unstable, especially when close to its melting point, 175 to 180°C, and its glass transition is below 5°C (McAdam et al., 2020a). Occurrence: It is a product of microbial secondary metabolism in the cells of bacteria. Bacteria strains include Ralstonia eutropha, Alcaligenes spp., Azotobacter spp., Bacillus spp., Nocardia spp., Pseudomonas spp., Rhizobium spp. and Ralstonia eutropha (McAdam et al., 2020a). Biopolymer blends/bionanocomposites: PHB possesses high crystallinity. However, its rigidity and low impact resistance limit its application. Therefore, it is usually blended with other polymers to enhance its mechanical properties and processibility. One instance is biodegradable blend nanocomposites that are produced from polylactic acid, poly(3-hydroxybutyrate) and cellulose nanocrystals (NCs) with dicumyl peroxide (DCP) as a crosslinking agent for 3D printing. In this case, PHB acts as a nucleating agent to induce PLA crystallization into a more ordered crystalline structure, attributed to its higher crystallinity compared to PLA (Frone et al., 2020). Due to the high crystallinity of PHB, it has been used as a fller for starch thermoplastic (TPS), and it resulted in better thermal stability compared to neat TPS (Florez et al., 2019).

1.9.3

POLYBUTYLENE SUCCINATE

Poly(butylene succinate) is a commercially available aliphatic polyester with many interesting properties, including biodegradability, melt processability and thermal and chemical resistance. Therefore, PBS is a cost-effective alternative to other polymers such as PLA, PBAT and PHB. It can be used either as a matrix polymer or in combination with other polymers such as PLA and PCL. Possible applications include food packaging, mulch flm, plant pots, hygiene products, fshing nets and fshing lines (Luyt & Malik, 2018). PBS is an aliphatic polyester, prepared by polycondensation of 1,4-butanediol and succinic acid via chemical synthesis. These monomers can be obtained either from renewable or fossil-based resources (de Matos Costa et al., 2020). The chemical structure of PBS is shown in Figure 1.26. Due to its

FIGURE 1.26 Chemical structure of PBS.

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semi-crystalline nature, thermal stability and melting point lower than that of other commercially available biodegradable polymers, PBS is an excellent candidate for the production of biodegradable flms for packaging. It has good barrier properties, as PBS’s oxygen permeability (PO) is about four times lower than that of low-density polyethylene (LDPE) and about double that of PLA. Occurrence: It can be produced from renewable resources such as sugars, glucose, starch and xylose via bacteria fermentation (de Matos Costa et al., 2020). Biopolymer blends/bionanocomposites: PBS exhibits low thermal stability, poor mechanical properties and slow crystallization rates. In order to enhance its properties, many studies have been done to incorporate inorganic materials or blend with other polymers. Inorganic materials such as clay and zeolite have been used to enhance the physical properties of PBS/ montmorillonite (MMT) nanocomposites (Hwang et al., 2012). Ultimately, PBS ionomers showed improved mechanical physical properties because of ionic interactions. The enzymatic hydrolysis behavior of PBS ionomers was drastically accelerated with increasing ionic content because of reduction in crystallinity and improved hydrophilicity. It was also blended with biodegradable polymers and PCL and incorporated with carbon nanotubes to achieve higher mechanical, thermal and electric conductivity compared to neat PBS (Gumede et al., 2018).

1.9.4

BIO-POLYETHYLENE

Biobased polyethylene falls into the category of polymers from renewable resources, considering the source of starting material (i.e. sugar cane), which can be obtained from renewable resources. It is produced through the fermentation of sugar to obtain bio-ethanol from plant resources. The bio-ethanol serves as the source for production. Bio-PE has similar characteristics to polyethylene derived from crude oil. Hence, the chemical structure, applications and recycling are all identical (Siracusa & Blanco, 2020). This also means that bio-PE is not biodegradable. From polymerization, there are basically two types of bio-PE: bio-low density polyethylene (bio-LDPE) and biohigh density polyethylene (bio-HDPE) (Siracusa & Blanco, 2020). The non-biodegradability of bio-PE is mainly due to the C–C and C–H bonds (𝜎 bonds), whose bond energy is on the order of 300–600 kJ/mol (Martínez-Romo et al., 2015). Occurrence: Starting materials obtained from renewable resources such as sugar cane, sugar beet and wheat grain (Siracusa & Blanco, 2020). Biopolymer blends/bionanocomposites: Since bio-PE is not biodegradable, many studies have incorporated biodegradable polymers to improve the biodegradability. This can be done through blending with biodegradable polymers or incorporation of fllers. One example is found in bioPE/PCL blends (Bezerra et al., 2019). An other instance is binary blends of bio-HDPE with PLA by melt compounding. The results showed that multifunctionalized vegetable oils and peroxides were needed to enhance

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the miscibility between bio-HDPE and PLA in order to achieve higher mechanical properties for packaging and kitchen utensil applications (Quiles-Carrillo et al., 2019).

1.10

SYNTHETIC POLYMERS FROM PETROLEUM-BASED RESOURCES

This section presents the chemical structure and characteristics of synthetic polymers from petroleum-based resources. In addition, several examples of using these synthetic polymers in biopolymer blends and bionanocomposites are detailed in this section. These include polycaprolactone, polyvinyl alcohol and polybutyrate adipate terephthalate.

1.10.1 POLYCAPROLACTONE Poly(ε-caprolactone) is a biodegradable aliphatic, which belongs to the poly(ahydroxyl acid) group and has been widely used in biomedical applications. This is mainly due to its biocompatibility and bioresorbability. Furthermore, the FDA has approved the usage of PCL for human implants, as toxicological assays show it is non-mutagenic and innocuous in animals (Espinoza et al., 2020). PCL is a polyester produced by ring-opening polymerization of epsilon-caprolactone, which is commonly derived from fossil carbon (Ali Akbari Ghavimi et al., 2015). Similar to most biodegradable polymers, the main chains of PCL consist of carbon. The chemical structure of PCL is shown in Figure 1.27. The thermal, physical and mechanical properties of PCL depend on its degree of crystallinity, which varies with molecular weight and reaches a maximum value of 69% in PCL. PCL exhibits high ductility and fexibility due to its low glass transition temperature of around –60°C. It can be easily processed through methods such as melt extrusion, flm blowing, injection molding and melt-spinning. PCL demonstrates excellent chain fexibility and can be synthesized in a range of molecular weights. While PCL is inherently hydrophobic, it is soluble in a wide range of aromatic, polar and chlorinated hydrocarbons but remains insoluble in aliphatic hydrocarbons, alcohols and glycols. The molecular weight of 15,000 g mol−1 means the material is brittle, and the molecular weight of 40,000 g mol−1 indicates that the material is soft and semi-crystalline in nature. PCL is able to degrade by

FIGURE 1.27 Chemical structure of PCL.

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hydrolysis in physiological conditions (such as in the human body), attributed to its ester linkages, but PCL has a longer degradation time than poly(lactic-co-glycolic) acid (PLGA) and PLA. PCL is thermally more stable than polylactic acid and completely degradable through enzymatic activities (Ali Akbari Ghavimi et al., 2015; Gumede et al., 2018). Occurrence: Raw material obtained from petroleum and can be synthesized by polycondensation of 6-hydroxyhexanoic acid as well as ring-opening polymerization of ε-caprolactone using Tin(II) 2-ethylhexanoate [Sn(Oct)2] as a catalyst (Gumede et al., 2018). Biopolymer blends/bionanocomposites: PCL undergoes degradation through the hydrolysis of its ester linkages under physiological conditions, making it a highly sought-after material for implantable biomaterial applications. Researchers have explored encapsulating various drugs within PCL beads to achieve controlled release and targeted drug delivery. Additionally, PCL is frequently blended with starch to create costeffective and biodegradable materials. Furthermore, low molecular weight PCL can be incorporated into other polymers to enhance their resistance to weathering. For example, PCL/PEG-PCL microspheres were added to estradiol, and it was proven to promote effcacy in encapsulation for drugs (Espinoza et al., 2020). On the other hand, PCL is often mixed with starch, as it was reported that PCL and starch mutually benefted whereby the PCL was able to alter the humidity sensitivity of starch, while starch was able to speed up the degradation rate of PCL, a function that is crucial in biomedical applications (Ali Akbari Ghavimi et  al., 2015). It is believed that the interaction between PCL and starch is formed through the hydrogen bonding between ester carbonyl of PCL and the hydroxyl group of starch.

1.10.2 POLYVINYL ALCOHOL In early years, the principal application of polyvinyl alcohol was in textile sizing. In recent years, PVOH have received much attention in biomedical devices because it is soluble in water, biodegradable and biocompatible and has a low protein absorption property and chemical resistance (Aslam et al., 2018). PVOH results from a hydrolysis reaction, which removes acetate groups from PVAc molecules without disrupting their long-chain structure (Aslam et al., 2018). The unique chemical feature is the presence of an OH group, which enables PVOH to interact with water and biopolymers with OH groups (Ali et  al., 2018). The chemical structure of the resulting vinyl alcohol repeating units is as shown in Figure 1.28. PVOH is soluble in water but insoluble in organic solvent and only sparsely soluble in ethanol. It shows compatibility with a number of polymers, and it can be easily mixed with various natural materials, which extends the range of its

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FIGURE 1.28 Chemical structure of PVOH.

applicability. It is biodegradable due to its excellent hydrophilicity, ascribed to the presence of hydroxyl groups on the carbon atoms, and is a semicrystalline polymer with high dielectric strength and high transparency (Aslam et  al., 2018; SettierRamírez et al., 2020). Occurrence: A hydrolysis product of polyvinyl acetate (Aslam et al., 2018). Biopolymer blends/bionanocomposites: PVOH has often been used for blending with other polymers. Blending PVOH with other polymers creates opportunities to improve its processibility, to modify physical properties and to reduce cost. For instance, PVOH is used as sizing agent and is able to provide greater strength to textile yarns, which causes them to be resistant to oils and greases. It is also used in adhesives and emulsifers to make water-soluble flm. Furthermore, it is easily modifed to form other polymers. PVA can be made into resins of polyvinyl butyral (PVB) and polyvinyl formal (PVF) with the reaction of butyraldehyde (CH3CH2CH2CHO) and formaldehyde (CH2O) (Aslam et al., 2018). Furthermore, the presence of OH groups serves to form hydrogen bonds with other polymers such as poly(vinylpyrrolidone) (PVP). One example is a blend of PVP and PVP doped with cerium nitrate developed by Ali et al. (2018), which is suitable in optical, electronic and organic semiconductor applications.

1.10.3

POLYBUTYRATE ADIPATE TEREPHTHALATE

Polybutyrate adipate terephthalate has been widely marketed as plastic packaging, shopping bags and garbage bags as an alternative to LDPE. Among the distinguished companies that have developed PBAT-based materials are BASF, Novamont, BIOTECH and KINGFA (Jian et  al., 2020). The aromatic portion of PBAT contributes outstanding physical properties, while its aliphatic chains facilitate degradation under various conditions, including soil degradation without requiring temperature control. This makes PBAT a highly promising material for disposable packaging products. The biodegradation of PBAT primarily involves hydrolysis catalyzed by microbial enzymes. During this process, the butylene adipate (BA) units in PBAT, especially the non-crystalline portion, tend to degrade more rapidly compared to the butylene terephthalate (BT) units with a crystalline structure. PBAT is an aliphatic-aromatic co-polyester of adipic acid 1,4-butanediol

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FIGURE 1.29 Chemical structure of PBAT.

and terephthalic acid from dimethyl terephthalate. The chemical structure of the PBAT polymer is shown in Figure 1.29. The real structure of PBAT occurs in a random block (Jian et al., 2020). PBAT shows biodegradability due to the aliphatic unit in the molecule chain, which is attributed to the cleavage of ester linkages and the ability to react with water, with its carbonyl group located in benzene rings. The mechanical properties of PBAT are more fexible than those of most biodegradable polyesters such as poly (lactic acid) and poly (butylene-co-succinate) and are similar to those of low-density PE. Due to the absence of structural order in PBAT, it has high fexibility and a low elastic modulus, which makes it ideal to be blended with other biodegradable polymers (Fu et al., 2020; Siracusa & Blanco, 2020). Occurrence: Polycondensation reaction of butanediol, adipic acid and terephthalic acid from the raw material petroleum (Siracusa & Blanco, 2020). Biopolymer blends/bionanocomposites: The properties of pure PBAT are not suffcient for most applications because it has higher production costs or lower mechanical properties when compared with conventional plastics. The development of a PBAT market will only be possible when production costs decrease or its properties are improved. Therefore, PBAT is often mixed with low-cost material like starch or PLA to decrease the end price and enhance the mechanical properties while maintaining the biodegradability of the composites. The biodegradation behavior of PBAT, PLA and PLA/PBAT blends in freshwater with sediment was affected by the content, aggregation structure, hydrophilia and biodegradation mechanisms of the components, where the crystallinity of PLA/PBAT composites increases with the degradation time, which is a result of the faster biodegradation of amorphous regions by microorganisms and enzymes, while elevated PLA content (25, 50, 75%) in composites leads to a higher increase in the O/C content ratio after degradation. After 24 months of degradation, an increase in the relative peak area proportion of C-O to C=O is observed (Fu et al., 2020; Jian et al., 2020). It is also incorporated with fllers, as done by Shankar and Rhim (2019), where the zinc oxide nanoparticles are reinforced in PLA/PBAT blends to enhance optical and mechanical properties along with antibacterial properties.

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1.11 POLYMER BLENDS Generally, blending is the method of thoroughly combining different materials to create a homogenous product. The terms blending and mixing are often used interchangeably, but there are some distinctions. Blending is typically a gentle method of combining materials, while mixing is usually a more vigorous process. Blending can help to enhance product quality by uniformly coating particles, diffusing liquids and fusing particles together. Therefore, blending is one of the methods that is commonly used for polymer modifcation in order to produce a new product with the desired properties (Imre & Pukánszky, 2013). Blending polymers is a good way to come up with novel materials with better properties (John, 2015). Altering surface properties such as friction coeffcient, fostering adhesion, adding color, improving stability, increasing output and gaining easy-opening features are just a few of the benefts of blending (Morris, 2016). In other words, by correctly selecting the component polymers, the properties of the blends can be manipulated according to their end use (Markovic & Visakh, 2017). In addition, there are some times when two natural polymers will coexist as a blend in nature, such as elastin and collagen in skin, where unique mechanical and structural properties can be demonstrated by such mixes (Sionkowska, 2015). According to Sperling (2000) Amos et  al. (1954) were credited with pioneering the feld of polymer blending, and it was further developed a generation after (Aylsworth, 1914). Amos et  al. (1954) invented high-impact polystyrene (HIPS). However, Ostromislensky was the frst to come up with the concept of rubber-toughened polystyrene in 1927 (Sperling, 2000). Without stirring, Ostromislensky dissolved rubber in a styrene monomer and polymerized it in order to impart impact resistance to the resulting material. With little toughening action, the result was a continuous rubber phase and a discontinuous plastic phase. Subsequently, Amos and his team made a substantial advancement by introducing agitation and shearing to the reaction between rubber and styrene solution. This innovative approach led to the evolution of HIPS as a widely used thermoplastic, primarily produced through continuous bulk processing. Today, HIPS stands as one of the top fve most-produced families of thermoplastic polymeric materials in terms of production volume (Bonilla-Cruz et al., 2013). Following this success, the development of acrylonitrile butadiene styrene (ABS) resins, as well as a wide range of other multicomponent polymer materials, occurred almost immediately. The feld of polymer blends has experienced an increasing rate of study over the past three decades due to the interest of researchers in blending methods to produce new materials (Sionkowska, 2011). Because people could make compositions with properties that homopolymers and statistical copolymers could not match, polymer blends and composites became a central part of polymer science and engineering (Sperling, 2000). Greater hardness and effect resistance, higher modulus, higher use temperature, wider temperature range of sound and vibration damping and so on are examples of such properties. In the same vein, Imre and Pukánszky (2013) also reported that blending of polymers is a technology that was developed in the 1970s or even earlier. On the other hand, according to Yu et  al. (2006), various starchpolyolefn blends were developed in the 1970s and 1980s.

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Blending a biopolymer with another polymer that does not have to be biodegradable is a benefcial way of changing its properties (Niaounakis, 2015b). Also, according a study reported by Imre and Pukánszky (2013), in the development of new polymeric materials, blending two or more polymers offers a great advantage by tailoring the properties of the polymeric materials over a wide range. Because of their theoretical and practical relevance, polymer blends have been extensively researched (Markovic & Visakh, 2017). Blending one biopolymer with another, such as PLA and starch, is the most commonly studied, but thermoplastic phenol formaldehyde resin/poly(e-caprolactone), chitosan/soy protein, PHB/cellulose acetate butyrate and PLA/poly (butylene succinate) are also worth mentioning (Imre & Pukánszky, 2013). Biopolymers are characterized as successful, long-term alternatives to traditional petrochemical-based products. However, their massive use has been hindered by some unsatisfactory material properties of these polymers, and the processability of nearly all pure environmentally friendly biopolymers is not equivalent to that of commercial thermoplastics (Niaounakis, 2015b). Besides, blending may be a much cheaper and quicker way to obtain the desired properties rather than the copolymerization technique (Tokiwa et al., 2009). A detailed examination of biopolymeric blends and composites, as well as their applications in diverse industrial sectors, is easy to fnd (Rogovina & Vikhoreva, 2006). Furthermore, blends can also help in the development of new low-cost and high-performance products. These new blends are transforming polymers from their sources into new value-added products (Yu et al., 2006). Polymer blends that have been produced can be utilized in a variety of applications due to their unique properties (Markovic & Visakh, 2017). Polymer blends are generally categorized as either homogeneous (molecularly miscible) or heterogeneous (immiscible) blends. However, according to Qin (2016), in a polymer blend or mixture, at least two polymers are combined to produce a new material with distinct physical properties, similar to metal alloys. Polymer blends can be categorized into three groups: immiscible, miscible and compatible polymer blends. In immiscible polymer blends, the constituent polymers occur in different stages, with different glass transition temperatures (Tg); in other words, they are blends that exhibit more than two phases. Usually, this kind of blend consists of two Tg values, since the two components of the blend are phase separated. Meanwhile, miscible polymer blends are blends that are frequently made from polymers with comparable chemical structures, resulting in a single-phase polymer blend. One glass transition temperature has been identifed. Also, compatible polymer blends are known as immiscible polymer blends, where the physical properties are macroscopically uniform owing to suffciently strong interactions between the constituent polymers. They are also blends that are useful and have inhomogeneity on a small enough scale that it is not noticeable when in use. To gain an advantage in processing or performance properties, the base and additive polymer should be compatible or miscible. Therefore, one of the most signifcant factors affecting the fnal polymer properties is the miscibility of the blend. The benefts of making miscible blends include single-phase morphology and mechanical property reversibility (Tokiwa et al., 2009). However, in cases where blended polymers exhibit incompatibility, resulting in the separation of the polymer mixture into

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distinct phases due to differences in molecular weight and viscosity, or when they display a coarsely dispersed structure, imperfections may become apparent on the surface of injection-molded products. In such situations, a surface exfoliation technique is often employed to improve the quality of the molded article (Niaounakis, 2015b). For example, miscible polymer blends include combinations like poly(styrene) (PS)/ poly(phenylene oxide) and poly(styrene-acrylonitrile)/poly(methyl methacrylate), while immiscible blends encompass poly(propylene PP)/PS and PP/poly(ethylene). According to Niaounakis (2015b), there are a few measurements that can be used to determine the miscibility of a blended polymer. A measurement of the optical clarity of a polymer blended flm, an appropriate measurement of mechanical characteristics and glass transition temperature (Tg) measurement using differential scanning calorimetry (DSC) are standard experimental procedures for assessing polymer blend miscibility. From an optical properties perspective, flms made from polymer blending can be seen clearly, which indicates that the blended polymers are miscible. Otherwise, if the blended polymers are immiscible, the resulting flm will look as if it contains foreign matter; that is, it is not optically clear. As indicated by Markovic and Visakh (2017), optical transparency is common in miscible mixes. Similarly, the mechanical properties of a blended polymer, such as tensile strength, are often in an intermediate phase between the blend components. As has been noted, the state of miscibility of a blended polymer can be determined by a few measurements, one of which involves measurement of the glass transition temperature by using DCS. This measurement is characterized under thermal properties. In addition, a miscible amorphous blend will have a single Tg intermediate between the homopolymer elements. Meanwhile, an incompatible or partly miscible blend would have several Tg values, where Tg will have intermediate values for partially miscible blends, referring to partially miscible stages rich in one of the components. A study reported by Ibrahim and Kadum (2010) shows that good miscibility is where, between two values of pure polymers, there is only one glass transition temperature. In their study, they blended polystyrene and acrylonitrilebutadiene-styrene in different ratios through a single-screw extruder, which is called the melt blending method. In addition, Macknight and Karasz (1989) also stated that a miscible polymer blend is one that meets the thermodynamic requirements of a single-phase system. For instance, if the blend is made up of two components, A and B, a single-phase mixture would form at a constant temperature and pressure. Also, Morris (2016) stated that the most basic blends can be created by combining ingredients in an extruder, which is used to convert the resin into a flm or coating. To achieve the desired properties in more complex blends, specialized screw designs or customized compounding equipment may be needed. Moreover, even when properties can be controlled by inserting specifc functional layers within a multilayer flm, blending is crucial for some applications, especially packaging. Blending may be used to tailor special properties, such as barrier performance, into a layer. Blending may be required to make the polymer stable enough to extrude or to ensure that the surface properties of the extruded product are satisfactory. Thus, the fnal blend characteristics will be determined by the thermodynamics and the thermal and rheological properties of the polymers, as well as the fow and stress history, which is process dependent.

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As eloquently stated by Aravind et al. (2009), blends of two different polymers are not always miscible or compatible with one another. Meanwhile, according to George et al. (2010), most polymer blends are immiscible to varying degrees, with complex phase morphologies that are dependent on the chemical nature of the constituents and their individual rheological properties, except for a few polymer blends that are thermodynamically miscible. In the same vein John (2015) found that in the case of immiscible blends, phase separation can occur, resulting in inferior compounded product properties. As a result, using graft and block copolymers to control phase morphology is essential. Even though immiscible systems are preferable in certain situations because each component keeps its own properties, miscible blends produce an average of individual properties. They are distinguished by a narrow interphase, weak interfacial adhesion and coarse morphology, all of which contribute to poor fnal properties (George et al., 2010).

1.12 METHODS OF POLYMER BLENDING As studied by Patil et al. (2016), polymer blends may be synthesized or processed using a variety of techniques. Each process has advantages and disadvantages. The methods that will be discussed are melt blending, mill mixing and fne powder mixing technology, the solution casting method, freeze drying, latex blending and interpenetrating polymer network technology. It is worth mentioning that the solution casting method is one of the methods that gained interest with most researchers, especially in a flm preparation (Patil et  al., 2016). In the same vein, Shundo and Ijioto (1966) also used some of the blending methods in their experiments with natural rubber and styrene-butadiene rubber blends, such as solution blending, latex blending, roll blending and Banbury mixer blending, in order to fnd which type of blending method is more effective in producing uniform blends.

1.12.1

MELT BLENDING

Preparation of polymer blends can be done through melt blending. Melt blending is the most common method of preparing polymer blends in practice, as it offers contamination-free polymer blend preparation processes (Patil et al., 2016). Due to the lack of organic solvents, melt blending is environmentally friendly. Melt blending integrates with today’s manufacturing systems like extrusion and injection molding. Melt blending has gained popularity as a result of its potential in industrial applications. Furthermore, according to Rane et  al. (2018), for producing a clay/polymer nanocomposite of thermoplastics and elastomeric polymeric matrix, melt blending method is the preferred approach. In addition, Kango et  al. (2014) indicated that melt blending is the most convenient and widely used technique for creating polymer hybrids with inorganic nanoparticles, such as semiconductor nanoparticles. Ma et al. (2006) also used melt blending to produce nanocomposites of silane modifed zinc oxide, ZnO and polystyrene resin to produce polystyrene resin nanocomposites with antistatic properties. In order to produce polymer nanocomposites through melt blending, inorganic nanoparticles are dispersed into the polymer matrix and undergo an extrusion process (Kango et al., 2014).

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In the melt blending process, individual components are processed and melted using specialized devices such as extruders and temperature controllers. Typically, extruders or batch mixers combine the blend components while they are still molten. According to Sadiku and Ogunniran (2013), the best way to disperse one polymer in another immiscible polymer is to use vigorous mechanical stirring in compounders at high temperatures when all components are in a molten state. The melt blending process is done in the presence of an inert gas such as helium or neon. To achieve a uniform mixture of all raw materials, the raw materials are subjected to a specifc chamber containing extruders. The temperature is raised to a desirable level, and all additional materials are melted as a result. Process conditions such as mixing duration, operating temperature and pressure, in addition to the composition of the constituents, are vital in achieving desirable blend properties. As eloquently stated by Giles et al. (2005), if the extruder temperature profle is incorrect, the product ingredients are incorrectly formulated, the melt temperature at the end of the extruder is incorrect, the puller at the end of the line is running at the incorrect speed, the cooling bath temperature is incorrect or any other incorrect operating condition occurs, the product that will be produced will not meet customer requirements. An incorrect setting at the start of the process may also result in an unacceptable product at the end of the line after a considerable amount of value has been added. According to Khan et al. (2019), this method is generally thought to be an excellent technique for the preparation of polymers, with the exception that it may appear to be too expensive at times.

1.12.2 MILL MIXING AND FINE POWDER MIXING TECHNIQUES Another method that is used to produce polymer blends is the mill mixing and fne powder mixing technique. Mixing components are mixed through milling and grinding in this simple, straightforward approach. This is accomplished through the use of various types of milling devices and grinders. The most common methods for achieving powder blending and particle size reduction are ball milling and attritor milling (Mehrotra, 2014). The raw materials are ground to produce the fnest powder, which is then blended together until the fnal state is reached, which is at the micro level. This is to make sure that the mixture is uniformly mixed. If the blend product meets the criteria, it is ready to be used for the next process where additional processing is needed in order to obtain the desired polymer blend products. However, if the powder cannot be reworked, the milling process may begin, or it may be scrapped. Bunbury mixers, also known as Master Mixers, are a widely used design for mechanical mixing of polymer ingredients, making them ideal for polymer blend synthesis (D. H. Killheffer, 1962). This mixer is made up of two counterrotating multilobed cams in a mixing chamber (Witt, 1984).

1.12.3

SOLUTION CASTING METHOD

The solution casting method is a polymer preparation that is favored by most researchers due to its easy and simple procedures (Khan et al., 2019). Solution casting is a simple and fexible procedure for making polymer flms in the laboratory. Simply, solution casting is the process of dispersing a chosen polymer component

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in a polymer solution and then casting the flm using standard coating procedures (Grothe et al., 2012). According to Mathew and Oksman (2014), solution casting is the oldest method of producing plastic flms and was invented by Eastman Kodak in the 19th century to produce photographic flms. The solution system’s primary components are polymer and solvent, but different additives may be added as well (Galiano, 2014). When it comes to preparing the casting solution, the polymer chosen is crucial. In fact, the polymer must be soluble in the chosen solvent at a concentration that is directly related to the fnal product application. To generate homogeneous composite flms, the polymer must have good solubility in the chosen solvent and excellent dispersion in the polymer solution. Khan et  al. (2019) explained this method briefy, where the frst step is that the components of the mixture are dissolved in a common solvent and vigorously stirred. Next, the selected solvent is used to dissolve the chosen polymers. It should be observed that solvent selection is critical, as it also plays a signifcant role in the solution casting procedure. Then, to get a homogeneous solution, the solution mixture is stirred for a certain amount of time. The stirred process can be done by using a homogenizer or magnetic stirrer or can be manually stirred by using a glass rod. A further step is the solution then is cast on a prepared mold. Finally, the end product is collected for further characterization. The procedure has the advantages of allowing the system to blend quickly without consuming a lot of energy, and the drying process of the solution on a surface does not cause any additional mechanical or thermal stress (Siemann, 2005).

1.12.4

FREEZE DRYING

Freeze drying, also known as lyophilization, is another polymer blending method. As eloquently stated by H. Zhang (2018), in the pharmaceutical, biological and agricultural industries, freeze-drying is a common drying method. Meanwhile, according to Ghalia and Dahman (2016), freeze drying is a method of removing residual solvent from a material in order to make a dry powder that can be loaded into a cell easily. When dealing with temperature-sensitive chemicals, this is the preferred drying method because it produces dry powders or cakes that are easy to store. The duration of drying the sample depends on the amount of liquid water present in the sample. In this method, the component polymers are quenched to a very low temperature during freeze-drying, causing the solution to freeze. Then, the solvent is removed by a sublimation process, which means it changes from solid state to a vapor phase. It is worth mentioning that this method has a few benefts, which are that heat-sensitive materials are less likely to degrade when dried at low temperatures, it is friendlier to antioxidant activity than other drying techniques and the physical form of the dry product can be attractive (Santiago & Moreira, 2020).

1.12.5

LATEX BLENDING

Latex is an important term in the polymer industry, and it has a particular meaning. It is used to create a very fne stable dispersion of polymer particles in any aqueous medium (Datta, 2013). Latex blending is thus a novel way for preparation of most common

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polymer blends and other polymerization methods (Khan et  al., 2019). Shundo and Ijioto (1966) stated that latex blending was an effective method in producing a uniform blend rather than other mechanical methods. However, these researchers reported that latex blending methods in preparation of polymer blends are not practical in the manufacturing industry. Latex blending is a dispersed mixture of two or more distinct types of latex components. Both kinds of particles contribute to the properties of the flm formed after the dispersion has dried. The properties of hard-soft latex blends have been studied by a number of scientists. Hard latex is made of a polymer with a Tg above room temperature, whereas soft latex is made of a polymer with low Tg.

1.12.6 INTERPENETRATING POLYMER NETWORKS The other method of preparation polymer blends is an interpenetrating polymer network (IPN). An IPN is polymer consisting of two or more molecularly interlaced networks that are not covalently bonded to each other and cannot be separated until chemical bonds are broken (Alemán et al., 2007). However, an IPN is not defned as a combination of two or more preformed polymer networks. The components in these systems are grown in such a manner that they are connected to each other, but not by any chemical bonds. As a result, special methods are often needed to create such polymer networks, as simple mixing of two or more polymers will not be suffcient to produce an IPN. There are a few types of IPN: semi-interpenetrating polymer networks (SIPNs), simultaneous interpenetrating polymer networks and sequential interpenetrating polymer network (Karak, 2012). A SIPN is created when a monomer polymerizes in the presence of a polymer. Alemán et al. (2007) defne a SIPN as a polymer made up of one or more polymer networks that are linear or branched polymers, with at least one of the networks being penetrated on a molecular scale. The polymer portion of the network can be split from the network in this case without affecting any chemical bonds. Meanwhile, a simultaneous interpenetrating polymer network is interpreted as a method for forming component networks at the same time, whereas a sequential interpenetrating polymer network is described as a procedure for forming a second component’s network after the frst component’s network has been established.

1.13 PROPERTIES OF POLYMER BLENDS Since the development of polymer blends has gained interest from researchers, understanding the performance or properties of the polymer blends is crucial. Each property has its own importance depending on the end use of the fnal product or application. Here are a few properties that are usually considered when preparing polymer blends: electrical, mechanical, thermal and optical properties. These properties can be controlled by using an appropriate amount of polymer components during polymer blends.

1.13.1 ELECTRICAL PROPERTIES Every property of a polymer blend depends on the behavior of polymer components incorporated into the blend. The properties of pure polymers can be enhanced by

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modifcation or through incorporation of fllers to produce the desired product. The demands of a specifc application force these polymeric materials to work under specifc conditions (e.g., chemical, mechanical and electrical) (Parameswaranpillai et al., 2014). It is worth mentioning that most researchers have an interest in investigating and reviewing conducting polymers because these types of polymer blends are increasingly being used in various electrical and electronic applications (Ameen et  al., 2006). Furthermore, conducting polymers are on the verge of being commercially viable (Xavier, 2014). Ameen et al. (2006) conducted an experiment on polyaniline-polyvinyl chloride (PANI-PVC) blends doped with sulfamic acid (dopant) in aqueous tetrahydrofuran to investigate the temperature dependence of direct current conductivity. To determine the impact of sulfamic acid (dopant) in the temperature range from 300–400K, the DC conductivity of PANI-PVC blended flms was tested. It was concluded that the conductivity of PANI-PVC blends increases with the amount of doped PANI.

1.13.2

MECHANICAL PROPERTIES

Mechanical properties are important properties that need to be considered when producing any product, especially products that need extra strength. These characteristics are essential when marketing a product, as customers will evaluate its strength before purchasing it. Abdul Khalil et al. (2019) stated that mechanical properties are evaluated by the stiffness and resistance to load exertion. The main fundamental parameters that are generally calculated to describe the mechanical properties of polymer blends are tensile strength, Young’s modulus, hardness and ductility (Khan et al., 2019). To evaluate their products, researchers typically use a variety of parameters and correlate them with mechanical properties (Pukánszky & Tüdõs, 1990). An article published by Hamad et al. (2016) studied the mechanical properties and compatibility of a PLA/PS polymer blend. They synthesized the blend by using a twin screw extruder and molded it using an injection machine. These researchers found that the mechanical properties of the polymer blend, tensile strength and elongation, increased when the PLA content increases. Thus, based on the mechanical property performances, they concluded that the PLA/PS blend has higher compatibility compared to other polyester/PS blends.

1.13.3

THERMAL PROPERTIES

Apart from electrical and mechanical properties, thermal properties are also important properties when preparing a polymer blend. The glass transition temperature Tg of a polymer blend is the parameter that is often considered for thermal industrial application, as it is the most common experimental technique. Also, critical temperature, heat capacity, heat defection temperature, fammability, solidus, thermal expansion, thermal conductivity and other parameters can be used to compare the thermal properties of various polymer blends (Khan et al., 2019). As mentioned, Tg is the major parameter when comparing thermal properties, as it will indicate whether the polymer blend is miscible or immiscible (Zainal & Chan, 2019). Ibrahim and Kadum (2010) studied the infuence of polymer blending on mechanical and thermal

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properties. The thermal properties are determined by using differential scanning calorimetry. The results from the work show a good indication for enhancing the miscibility of polymer blends, as for the two values of pure polymers, there is only one glass transition temperature.

1.13.4 OPTICAL PROPERTIES One of the most fundamental and essential properties of polymer blends is their optical properties, which aid in extracting a variety of knowledge about the blends (Khan et al., 2019). After the launch of transparent ABS in the market, optical properties have gotten a lot of attention from the scientifc community and industry. Although there has been limited progress in this feld, these properties are critical to remember when evaluating the suitability of a blend for a specifc application (Xavier, 2014). Transparency is one of optical properties that infuence consumer acceptance of the product. This is due to the fact that a high level of transparency results in a strong visual display of items. Transparency is correlated to the interaction of radiation between the substances. Takahashi et al. (2012) studied the optical properties for an immiscible blend of poly (methyl methacrylate) and ethylene–vinyl acetate copolymer (EVA). It was stated that when the difference in refractive index between the two components is small, PMMA/EVA exhibits good transparency at room temperature. Furthermore, Errico et al. (2006) reported that a polymer blend system consisting of EVA-g-PMMA and PMMA became opaque at high temperatures and transparent at room temperature, which shows that transparency is dependent on the difference in temperature.

1.14

BLENDING BIOPOLYMERS WITH OTHER BIOPOLYMERS

The combination of biopolymers exhibits properties that differ or have some improvements from those of their individual polymer counterparts, making them a preference among most researchers. According to Bonilla et al. (2014), flms produced by blending polymers typically have different physical and mechanical properties compared to flms made of individual components. Therefore, the combination of two different natural biopolymers is favored by most researchers because it is biodegradable, low in cost and widely available in nature. Researchers focused on renewable resources to create biodegradable materials in order to protect the environment and reduce emissions caused by oil-derived plastics (Mohanty et al., 2000).

1.15

BLENDING OF NATURAL BIOPOLYMERS WITH OTHER NATURAL BIOPOLYMERS

Chitosan, starch and alginate are some examples of natural biopolymers. Chitosan is mostly incompatible with commercial polymers, but it has a good outcome when blended with other biopolymers in a variety of application such as food packaging, biomedical, conductive material and metal complexation resin (Heeres et  al., 2013). Chitosan has many excellent characteristics such as biodegradability, biocompatibility, bioactive, nontoxicity, antibacterial properties, gel-forming properties,

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hydrophilicity and selectively permeability (Rajeswari et al., 2020). Besides chitosan, starch is also a commonly used biopolymer in many applications. According to European Bioplastics (2020), statistics show that approximately 18.7% of starch blends were produced in 2020. Low cost and wide availability make starch a popular biopolymer in many applications (Heeres et al., 2013). Alginate is a hydrophilic polysaccharide commonly found in the cell walls of brown algae that when hydrated forms a viscous gum (Pawar & Edgar, 2012). Alginate is a new type of industrial food coating that binds divalent metal ions to form a gel (Kulig et al., 2016). Food packaging is one of the many industrial applications for biodegradable materials. To improve food safety, chitosan was chosen because it has antimicrobial properties and can provide edible protective coating (Dutta et al., 2009). The other biopolymer used is starch to reduce water absorption and increase mechanical properties (Alix et al., 2013). Good flm-forming capacities in chitosan and starch contribute to the formation of the composite flm (Talón et al., 2017). According to Tripathi et al. (2008) chitosan–starch has been formulated, resulting in a potential food packaging application by the preparation of ferulic acid incorporated with a starch–chitosan blend flm. Another study shows a chitosan–rice starch blend flm was prepared by Brodnjak and Todorova (2018) using chitosan, rice starch and other materials, that is, malic acid and glycerol, with ultrasonic treatment. Several properties of chitosan–rice starch blend flms were investigated for comparison with chitosan and rice starch flm. The moisture content was increased due to the presence of rice starch, which can produce a highly crosslinked system and therefore prevent water molecules from penetrating the composite flm. Water vapor permeability (WVP) also decreased because of the addition of glycerol. The tensile strength and elongation at breaks were increased due to acoustic activation in an ultrasonic bath. Also, the mechanical resistance was improved and caused homogeneity of the surface. In overall, a chitosan blend with rice starch improved the characteristics of the composite flm. Furthermore, biopolymers are useful in tissue engineering to produce a biodegradable scaffold that acts as a temporary skeleton for accommodating and stimulating new tissue growth (Li et  al., 2005). Before biodegradable scaffolds were introduced, bioactive ceramics and polymers were developed as a tissue engineering scaffold, and the drawbacks are that they are inherently brittle and have low biodegradation rates (Li et al., 2005). To overcome these problems, biopolymers were used because they have high biodegradation rates and good mechanical properties. The biopolymer used is chitosan and alginate. Chitosan is known to have good biocompatibility, biodegradability, low toxicity and the ability to be fabricated in different shapes, while alginate is commonly used in bone tissue engineering because it is biocompatible, hydrophilic and biodegradable under normal physiological conditions (Wang et al., 2017). As well as in tissue engineering, the combination of chitosan and alginate has various applications due to its properties of polyelectrolyte complex fbers, beads, nanolayered polyethylene terephthalate (PET) flms, drug-loaded membranes and multilayers (Kulig et al., 2016). Wang et al. (2017) proposed a preparation to blend chitosan with alginate, and the product was called a chitosan–alginate hybrid. The combination between these two polymers resulted strong ionic bonding between the

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amino group of chitosan and carboxyl group of alginates. The scaffold produced had a highly porous and interconnected pore structure, making it suitable for cell attachment, proliferation and tissue growth and allowing nutrients to pass through and metabolites to exchange (Wang et al., 2007). Moreover, a chitosan–alginate hybrid is compatible with proliferation of olfactory ensheathing cells (OECs) and neural stem cells (NSCs), which are cells for regrowth and self-renewal. Overall, a chitosan–alginate blend is very suitable for use as a scaffold in tissue engineering. Many researchers are interested in the combination of cellulose and starch. Both cellulose and starch are natural biopolymers from plants and have similar chemical structures. Biodegradable properties, low price and abundant availability of starch and cellulose are the reasons many experiments with cellulose–starch blends have been performed. The unique characteristics of the product of blending cellulose and starch can be used in many applications, such as food packaging, biomedical and environmental (Shang et al., 2019). From Arvanitoyannis and Kassaveti (2009), various types of starch–cellulose blends show excellent mechanical properties, thermal properties, water vapor transmission rate and gas permeability. Generally, blending a cellulose derivative or starch derivative has wide application compared to natural cellulose and starch, but a natural cellulose–starch blend shows promising characteristics with numerous applications. Miyamoto et  al. (2009) investigated the structure and properties of cellulose– starch blend flms regenerated from aqueous sodium hydroxide (NaOH) solution. CEKICEL, a cellulose–corn starch blend, was produced as a food material from aqueous NaOH solutions on a commercial scale for the frst time. However, limited information is available regarding its properties and structure (Hisano et al., 1991). Miyamoto et al. (2009) published a paper on the structure of a cellulose–corn starch blend flm made from a cellulose–corn starch aq. NaOH solution with varied proportions of polymer followed by coagulation of sulfuric acid. The structure and properties were evaluated by using scanning electron microscopy (SEM), wide-angle x-ray diffraction (WAXD), dynamic viscoelastic measurement and other equipment, and the result was that the average pore size of cellulose–starch blend flms increased from 1 to 6 m as the starch content increased. These pores existed separately from one another, which may have contributed to the blend flms’ high water and oil absorbency. Another result was that the crystalline regions of the blends were incompatible, while in the amorphous regions, cellulose and starch were miscible to some degree. Overall, Miyamoto et al. (2009) concluded that the structure and properties of the cellulose–starch blend were scientifcally important, and it was expected to be widely used in food material. Another study on cellulose–starch was conducted by Shang et  al. (2019) using cellulose–starch hybrid flms plasticized by aqueous ZnCl2 solution. The objective of that paper was to learn how cellulose and starch interact in ZnCl2 solutions as two biopolymers with identical structures and how these interactions affect the structure and properties of the cellulose–starch blend hybrid materials. The experiment was carried out by preparing cellulose–starch hybrid flms plasticized by an aqueous ZnCl2 solution, varying the starch/cellulose ratios. The structure and mechanical properties of the flm were inspected by using SEM, X-ray diffraction (XRD) and Fourier-transform infrared. The result shows that the mechanical properties, that

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is, tensile strength and elongation of the cellulose–starch hybrid, were improved, while the crystallinity of starch was reduced. Shang et al. (2019) also notices that in cellulose-rich hybrid flms, although chemical interactions were not affected, a higher starch content decreased the material properties. Besides starch-cellulose blends, there is also a compatible blend between starch and microcrystalline cellulose. MCC is a naturally occurring substance made from partly depolymerized and purifed cellulose (Trache et  al., 2016). Excipient is a substance that serves as the medium for a drug or other active substance, and it is important in industrial pharmacy. Co-processed excipients combine two or more excipients using a suitable manufacturing process that will not change the chemical structure in the excipient (Patel & Gohel, 2016). Co-processed excipients help to solve the drawbacks that can arise by using general-grade excipients (Mamatha et al., 2017). Limwong et al. (2004) blended rice starch (RS) and MCC to produce a co-processed excipient for direct compression. The method used was a spray-drying technique. Spray-drying is a process that uses a hot gas to rapidly dry a liquid or slurry into a dry powder. The spray-drying technique can overcome conventional emulsion stability issues during storage and distribution of dry products without using harmful organic solvents (Li et al., 2019). Limwong et al. (2004) carried out an experiment with a combination of RS and MCC by varying the RS:MCC ratio. They concluded that an RS and MCC ratio of 7:3 was the best combination that exhibited the result of the co-processed excipient with high compressibility, good fowability and self-disintegration. Builders et al. (2010) also published that blending starch and MCC can produce excipient. This time, they blended MCC and maize starch (Mst), resulting in a novel multifunctional excipient MCC-Mst. A novel excipient means an excipient that is being used in a drug product for the frst time. A multifunctional excipient defned as an excipient that provides added functionalities to the formulation or that has several functions in the formulation (Bhor et al., 2014). A multifunctional excipient provide several benefts, including lower production costs and fewer steps in the manufacturing process (Nachaegari & Bansal, 2004). The method used by Builders et al. (2010) to produce the multifunctional excipient MCC-Mst was known as compatibilized polymer blending by mixing colloidal dispersions of MCC and chemically gelatinized Mst at controlled temperatures. The result shows that in terms of direct compression capacity and disintegration performance, the MCC–Mst composites were comparable to MCC and Mst. The multifunctional MCC-Mst combination demonstrated excellent performance in terms of disintegration effciency and loading capacity when incorporated into oral tablet formulations designed for the rapid release of active pharmaceutical ingredients (APIs) using the direct compression method. Pharmaceutical excipients are substances that are added to a pharmaceutical dosage form such as a capsule to help with the production process; to protect, sustain, or enhance stability; or to improve bioavailability or patient acceptability (Haywood & Glass, 2011). Hence, capsules also play an important role in pharmaceuticals for drug delivery. A capsule is an edible package made of gelatine or another suitable material that is packed with medicine to create a unit of dosage (Hadi et al., 2013). Gelatine is a biopolymer that is commonly used in a number of industries due to

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its biocompatibility and biodegradability. Gelatine also has properties such as high water solubility, non-toxicity, high mechanical strength, elasticity in dry state and moisturizing by binding a signifcant volume of water. These excellent properties mean it is used in the pharmaceutical, photography and cosmetic industries (Das et al., 2017). Many scientists have tried to obtain a suitable capsule that can be used in drug delivery. Zhang et al. (2013) published a paper on developing capsule materials by using gelatine–starch blends. They blended gelatine with corn starch, and poly (ethylene glycol) was used as a plasticizer and compatibilizer by varying the gelatine/ starch ratio. The result shows that the blend with gelatine/starch in a ratio of 50:50 exhibited a good flm and capsule. The properties of the blend consist of high viscosity. The transparency and toughness of the blends is increased due to the existence of PEG. The mechanical properties of the capsule were also investigated and showed an increase in tensile strength and decrease in elongation. The capsule seemed to be more rigid and brittle. The blend of starch and gelatine is not only for capsules; it can also be used as an alternative to plastics in agriculture, as reported by Rosseto et al. (2019). Plastics used in agriculture are largely derived from synthetic petrochemical polymers. These products need a proper waste management system at the end of their lives, since they can pollute the soil and create high-cost environmental impacts for manufacturers (Blanco et al., 2018). To overcome this problem, many researchers have come out with biodegradable plastic. Biodegradable plastic is often made from low-cost, renewable resources that are abundant in nature, such as starch, gelatine, cellulose and other biopolymers (Cazón et al., 2017). There are many challenges to produce biodegradable plastic; for example, it is very costly compared to conventional plastic, and in term of properties, commodity plastic shows better properties (Imre & Pukánszky, 2013). Hybridization of starch and gelatin has garnered signifcant attention in various research studies due to their advantages, including biodegradability and wide availability among consumers, which in turn leads to reduced raw material expenses (Rosseto et al., 2019). The process employed to combine starch and gelatin is known as extrusion. Extrusion is a mechanical procedure involving the feeding of polymers in various forms, such as granules, powders, fakes and pellets, through an extruder screw (Rosato, 1998). Rosseto et al. (2019) showed many methods of combination for starch and gelatine that can produce biodegradable plastics, such as using starch with the addition of hydrocolloid to protect granules from shearing during the manufacturing process; retain moisture; and reduce the blend’s syneresis, water solubility and water absorption. Also, the addition of maize starch to gelatine flms can increases the thickness, transparency and mechanical strength of the flms and also enhance their structure, all of which increases the flms’ applicability. The authors also stated that when blending cassava starch and gelatine, higher gelatine concentrations increased the water vapor permeability, and mechanical strength values decreased the opacity of the flm. In agricultural production, rapeseed was coated with a biodegradable liquid flm made of oxidized maize starch and gelatine from leather scraps. The flm was found to increase the rate of rapeseed survival and yield and is suitable for agricultural

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production. The flm has strong water absorption and retention properties, allowing plants to thrive in the most basic conditions. In addition, the ability of agricultural mulch flms made from native and phosphorylated corn starch, with and without chitosan surface functionalization, was assessed. Rosseto et al. (2019) concluded that the properties demonstrated by the various compositions studied may allow biodegradable polymer flms to be used in agriculture, particularly in mulching, because mulching plastics are normally replaced every crop cycle, whereas biodegradable ones may mix with the soil.

1.16 BLENDING OF SYNTHETIC BIOPOLYMERS WITH OTHER SYNTHETIC BIOPOLYMERS Blending synthetic biopolymers with other synthetic biopolymers has also gained much attention from researchers and industry. This is because the use of biopolymers in blends can reduce cost and be environmentally friendly due to their properties. Nanda et  al. (2011) studied the effect of process engineering on PLA/PHBV blend properties. The biopolymer blend was prepared by the melt blending technique, as this technique is an effcient method for polymer blending. In this study, it was reported that the addition of PLA to PHBV signifcantly increased the mechanical properties, the tensile strength and Young’s modulus of the blend. The increase of strength and modulus of the blend may be due to the properties of PLA, which is high in strength and modulus, as has been reported in other literature (Huda et al., 2006; Richards et al., 2008). Another increase was found in elongation at breaks of the blend with the increase in PLA content. However, the addition of PHBV content to the PHBV/PLA blend was shown to lower the crystallization temperature of PLA. Furthermore, dynamic mechanical analysis showed that the storage modulus was improved with the addition of PLA into the blend system. The authors concluded that this polymer blend can replace petroleum-based polymers due to its eco-friendly properties. Other recent studies on PLA/PHBV blends for various applications have also been reported in other literature (Ferreira & Duek, 2005). Jiang et al. (2006) proposed a study of a biodegradable blend of polylactide with poly (butylene adipate-co-terephthalate). Combining PLA with PBAT is a safe way to enhance PLA’s properties while preserving its biodegradability. PBAT is a polymer that is fexible and fully biodegradable (Chiu et  al., 2013; Pietrosanto et  al., 2020). The PLA and PBAT blend was made by the melt blending technique, as both polymers were fed into a twin-screw extruder. The authors found that the mechanical properties (tensile strength and modulus) of the polymer blend decreased with the addition of PBAT content. Nonetheless, elongation at breaks and toughness of the blend gradually increased. This is due to the lower tensile strength and modulus of PBAT compared to PLA. A study of miscibility of the blend was done through differential scanning calorimetry analysis. The result showed that the blend was not miscible, as a two-phase system with two glass transitions was obtained even with the addition of PBAT content into the blend. This indicates that there is poor intermolecular interaction between PLA and PBAT. Another result obtained from this study is that the blend viscosity and melt elasticity increased with an increase in PBAT concentration, as PBAT has higher elasticity and viscosity compared to

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PLA. Other literature has proposed the effect of compatibilizers on the properties of PLA/PBAT biopolymer blends (Kumar et al., 2010; Al-Itry et al., 2012; Al-Itry et al., 2014). There is much literature on the investigation of blends of PLA with polycaprolactone biopolymers. PCL is a biodegradable (Takayama & Todo, 2006), biocompatible and eco-friendly polymer that is prepared by ring-opening polymerization of ε-caprolactone with the aid of a catalyst. PCL is primarily used in medical applications (Mofokeng & Luyt, 2015). Recently, PCL has been used in numerous applications such as packaging, agricultural flm and manufacturing of disposable material (Malinowski, 2016). One study is from Matta et al. (2014), where the authors discuss the characterization of PLA/PCL blends with various concentrations of PCL. In the study, they produced blends by using a Hakee Rheomix, where the mixer blended the composition of polymeric materials in same processing treatment. From the experiment, it was found that the tensile strength and modulus of the blend decreased due to the formation of PCL spherulites acting as stress concentrators in the PLA matrix. Meanwhile, the viscosity of the blend increased with an increasing PCL concentration, as PCL has higher shear viscosity than PLA. Moreover, it was investigated that the PLA/PCL blend is not miscible, as phase separation in the blend was observed. Similar results of immiscibility of PLA/PCL were reported in Simoes et al. (2009), Gardella et al. (2014) and Urquijo et al. (2015). Another study of synthetic biopolymer blends is from Yang et  al. (2019) and detailed the production of PLA blended with poly (butylene succinate adipate) (PBSA) flms containing active compounds to improve the properties of food packaging and its shelf life. Essential oil is often used as active ingredient in polymeric blends for designing active food packaging systems (Alboofetileh et al., 2014; Ma et al., 2017; Chen et al., 2019). The addition of active compounds into polymer materials is one of the steps taken to preserve food freshness as well as to maintain the food quality. PBSA is a copolyester that is synthesized by polycondensation of 1,4-butanediol in the presence of adipic acids (Ahn et al., 2001). In this research, the authors created innovative active flms using the extrusion-casting technique. The experimental fndings revealed enhancements in two signifcant mechanical properties: tensile strength and elongation at break. Furthermore, the subsequent evaluation, involving the release rate of active compounds from the flm into a food simulant and the flm’s antioxidant effciency, also yielded favorable outcomes. As mentioned, one of the main goals is to investigate the shelf life of food by using active flms. Thus, the goal was achieved, as the shelf life of the food was extended while preserving the food quality. This is because the antibacterial and antioxidant properties of the PLA/PBSA flms were improved when the active compound was released into the food. From this experiment, it was concluded that this biodegradable active flm can replace non-biodegradable flm in food products, particularly in food packaging. Poly(hydroxybutyrate) (PHB) is a biodegradable biopolymer that also has been used in various types of polymer blending. PHB has been produced by microorganisms under certain conditions (Bharti & G, 2016; McAdam et al., 2020b). PHB is a polymer with a highly crystalline structure, where its crystallinity is above 50%. PHB has been used widely in medical applications, such as in tissue engineering as

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scaffolds and in drug carriers (Williams et al., 1999; Zinn et al., 2001; Phillip et al., 2007), and also in packaging applications (Rosa et al., 2004). Arrieta et al. (2017b) reviewed a polymer blend of PLA with PHB for food packaging through the melt blending technique. It was stated that the addition of PHB improved the performance of the pure PLA. To improve the processability of the PLA/PHB blend, plasticizer is often added. Moreover, the incorporation of PHB into the blend as a nucleating agent for PLA increased the PLA crystallinity, thus improving the barrier properties of the PLA/PHB blend. Recent developments of PLA/PHB formulations with improved properties are promising to replace petroleum-based polymers that have been used in food packaging.

1.17

BLENDING OF NATURAL BIOPOLYMERS WITH SYNTHETIC BIOPOLYMERS

Biodegradable synthetic polymers, in general, have many benefts over natural polymers. They are also biopolymers that, due to their biocompatibility and biodegradability, have the potential to replace petroleum-based polymers in a variety of applications, especially in the biomedical feld, as a result of their modifcation and also can be fabricated in a variety of shapes (Tian et al., 2012). For instance, Tănase et al. (2015) reported blending polyvinyl alcohol and starch in their paper. PVA is a synthetic biopolymer that is biodegradable, biocompatible and soluble in water due to the presence of polar alcohol groups, which can form hydrogen bonds with water (Kanatt et al., 2012). Even though PVA is biodegradable, many researchers make an effort to improve its biodegradability by blending it with other polymers. Meanwhile, starch is a natural biopolymer that is frequently used in non-food applications, primarily in paper making and textiles (Vilaseca et al., 2007), as it can be easily found and is abundant. More recently, starch has been used as raw material in producing biodegradable flm. Despite its advantages, starch-based materials are known to have some drawbacks, such as lower mechanical properties, poor processibility and extreme sensitive to water (Niranjana Prabhu & Prashantha, 2018). A study by Tănase et al. (2015) in producing biopolymer blends was proposed. As mentioned, the researchers used PVA and starch for their polymeric blends, such as for food industry and packaging application. In this research, they used PVA as a thermoplastic matrix and starch as a biodegradation agent. They also used glycerol as plasticizer to increase the fexibility behavior of starch and PVA since starch is not fully compatible with PVA. The fnal product of the blending was a flm and was characterized in a few areas, such as optical and permeability properties, which are the main properties in packaging. For optical properties, it showed that flms with starch exhibit low light transmission compared to PVA/glycerol flms, which means that flms with starch incorporation have high opacity. Thus, this shows that the PVA/starch flms have excellent barrier properties, which is the most important category of properties for food packaging, as it can protect food from UV lights. Next is permeability, which also a dominant property for food packaging. The permeability of the flms is increased along with an increase in starch. It was stated that the permeability properties are affected by the sample’s crystallinity, where the crystallinity decreases when the permeability increases.

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From this paper, it can be concluded that these biopolymer blends have good physical properties, which makes them suitable for packaging application. In the same vein, Bonilla et al. (2014) also produced a biodegradable flm based on PVA and chitosan for food packaging products. Yu et al. (2018) stated that PVA and chitosan, which are biocompatible and have strong flming formation, have received a lot of attention as biodegradable polymers because of their environmental benefts. According to Chiellini et al. (2003), PVA has been widely used in the preparation of polymeric blends and composites with other biopolymers. Meanwhile, chitosan flms have a lot of potential as active packaging materials because of their low oxygen permeability and antimicrobial activity (Kanatt et al., 2012). Theoretically, chitosan is miscible with PVA, as they can form hydrogen bonds due to the presence of hydroxyl and amine groups in chitosan (Chen et al., 2008). As for that, by using the polymer blending process, PVA was blended with chitosan to improve the physical properties of chitosan-based materials (Chen et al., 2007). In a paper by Bonilla et al. (2014), it was stated that the addition of chitosan in PVA flm reduced flm stretchability, which makes it resistant to fracture compared to pure biopolymer flms. This is due to the formation of inter-chain bonds that reinforce cohesion between the polymer networks but limit the polymer chain slippage. Furthermore, according to the researchers, the addition of chitosan into PVA flms reduced the UV light transmission of the blended flm. Lower transmission of UV light can give a blended flm good barrier properties, which can protect the food inside the packaging from lipid oxidation induced by UV light. On the other hand, Tang et  al. (2012) discussed biopolymer blending based on starch and polylactic acid. PLA is a biodegradable synthetic polymers, which is the most common polymer that has been used in recent development of polymer blending (Gunatillake & Adhikari, 2003). Among the families of synthetic polymers, PLA is often used to blend with starch to increase the performance of the product, such as to increase the biodegradability and reduce the cost (Tang et al., 2012). The frst polymer blend system based on PLA and starch was patented by Ajioka et al. (1995). They produced polymer blend flms by using a sealed mixer at the frst step and then using a hot-pressed procedure, resulting in a smooth and translucent flm. In recent years, several studies have been done based on this kind of polymer blending. Jun (2000) studied the performance of PLA/starch blending with a combination of reactive agents during the melt extrusion process. Diisocyanates such as toluene diisocyanate (TDI), 1,6-diisocyanatohexane (DIH) and 4,4′-methylenebis (phenylisocyanate) (MDI) have been used as crosslinked agents. The agents increased the tensile strength of the polymer blend. The use of DHI crosslinking agents gave a higher tensile strength compared to other agents. This is because the DHI agent has a long chain structure and is more fexible that other crosslinking agents. However, the elongation at breaks of the blends did not show a signifcant increase compared to tensile strength even with the addition of reactive agents. This shows that the plasticizing properties of the polymer blend cannot be improved. Jang et al. (2007) characterized the thermal properties and morphology of a PLA/starch compatibilized blend. In the preparation of PLA/starch blends, reactive compatibilizers such as maleic anhydride (MA) and maleated thermoplastic starch (MATPS) are used to enhance interfacial adhesion. They found that MA is a good compatibilizer for PLA/starch

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blend systems, while MATPS is not. MA has shown good interfacial morphology when it has been incorporated into a blend system. As for the thermal properties, the crystallinity of PLA/starch blends improved with MA as compatibilizer. The increase of crystallinity is due to the effects of plasticization by MA. Other natural and synthetic biodegradable polymer blends have gained much less recognition than starch-based biodegradable polymer blends. Claro et al. (2016) studied a comparison of PLA/chitosan with a PLA/cellulose acetate blend for packaging applications. Their goal was to produce biodegradable flms with improved properties without the addition of an additive or plasticizer. It may perhaps be observed without straying too far afeld from the main focus that cellulose acetate is also a biopolymer that is synthesized from an esterifcation reaction of cellulose with acetic acid and anhydrous acetate by using sulfuric acid as a catalyst (Edgar et al., 2001). Per its name, cellulose acetate is a cellulose derivative that has antimicrobial properties. From their study, the authors found that PLA/chitosan flm had the greatest increase in mechanical properties, where the elongation at breaks of the blended flm increased compared to pure PLA flm. This shows that PLA/chitosan flm is the best option to be used in packaging applications compared to PLA/cellulose acetate flm. On the other hand, the PLA/cellulose acetate blend did not show improvement in mechanical properties; the tensile strength and elastic modulus resulted in the lowest values. Despite this drawback, the PLA/cellulose acetate blend still showed an increase in elongation at breaks. Based on the result, the authors proved that PLA/ chitosan and PLA/cellulose acetate blends do not need any additives or plasticizers in their blends for use in packaging applications. Another study on biopolymer blends was by Sionkowska (2003), who prepared a collagen/PVP flm by using the solution casting method. The work was done to investigate the interaction between collagen and PVP in the blend as well as to fnd the miscibility state of those blends. It was found that collagen and PVP have good interaction with each other. This can be seen by the result obtained from viscometry, DSC and fourier transform infrared spectroscopy (FTIR). Based on viscometric data, it was shown that the collagen/PVP blend is miscible, as the criteria for miscibility were detected by viscometry. Meanwhile, a DSC test is the common method to determine the miscibility of polymer blends. The main parameter of miscibility is the glass transition temperature (Tg). DSC showed that the glass transition temperatures for the blends and single components were distinct. On the other hand, the FTIR test also showed that collagen and PVP had good interaction, as FTIR could detect the intermolecular interaction between those two polymers. From the test, it was found that the position of amide band A and amide I band of collagen moved to a higher frequency after blending with PVP. This shows that formation of hydrogen bonding occurred between collagen and PVP. The author has also investigated other collagen-based polymer blending in her studies (Sionkowska et al., 2009).

1.18

SURFACE MODIFICATION OF BLENDING

From previous topic, it can be seen that many types of biopolymer blends can be tailored for specifc applications. The examples show that the result of compatible

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blending between two polymers can produce a new material with desired characteristics. However, there are some biopolymer blends that are immiscible with each other, meaning another step must be taken, known as surface modifcation (Imre & Pukánszky, 2013). By performing surface modifcation, the blends between two biopolymers become compatible. Surface modifcation is making many blends compatible, and it increases the advantage of biopolymer blends that can be used in various applications. As matter of fact, surface modifcation can not only be used in polymers, but it has been used in many ways due to polymer surface properties limiting their intended wide range of applications (Nemani et al., 2018). Fabbri and Messori (2017) stated that the most popular goal of surface modifcation is to modify the outermost layer of a polymer by introducing functional groups onto the surface to improve certain properties such as barrier properties, adhesion and wettability. The surface of the polymer means the frontier between two different media, characterized by a certain thickness that refects a gradient of properties (Mincheva & Raquez, 2019). Generally, surface modifcation methods have been classifed based on the characteristic original material being modifed, such as physical, chemical and biological characteristics (Fabbri & Messori, 2017). The easiest, low-cost, scalable way of surface modifcation is using physical methods. It provides more robust and abrasionresistant polymer surfaces. It is also eco-friendly because no chemicals are needed in this process (Ozdemir et al., 1999). Besides physical methods, there are chemical methods that use chemicals to modify the surface of a component. Unlike physical methods, chemical methods can enhance the properties of a component without altering the surface roughness. Most chemical surface treatment methods use wet procedures in which the polymer is dipped, coated or sprayed with a chemical to improve its surface properties (Mitra & Saha, 2013). Recent advancements in the utilization of biopolymers highlight the necessity for the introduction of a new category of polymer modifcation techniques, specifcally those rooted in biological approaches. Biological methods involve the use of proteins, peptides, ligands, receptors and other fundamental biomolecules, while additional compounds like drugs and lipids can be incorporated onto the material’s surface. These methods encompass physical adsorption, self-crosslinking and chemical conjugation. When it comes to blending, there are several popular strategies for surface modifcation in polymer blends, including copolymerization, grafting, trans-esterifcation and the utilization of reactive coupling agents.

1.18.1 COPOLYMERIZATION Copolymerization is an effective method used for tailoring polymers. Copolymerization involves polymerizing two separate monomers simultaneously, with the intention of combining both of the structures into a single polymer chain. This greatly broadens and diversifes the range and variety of properties of copolymer molecules, allowing for the incorporation of desirable properties from various monomer units (Scott & Penlidis, 2017). A study by Graebling and Bataille (1994) proposed an experiment on polypropylene/poly(hydroxybutyrate) blends with the presence of PP-g-PHB copolymer. The copolymer of this blend was prepared based

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on a three-step procedure, alchoholysis of PHB, grafting of styrene and maleic anhydride on PP and the reaction between the polymers obtained. The morphology of a PP-PHB blend changes signifcantly when just the copolymer is present, as the PHB polymers tend to disappear into the PP matrix. Also, for mechanical properties, tensile strength showed an increase in yield stress but a decrease in break properties with the presence of the copolymer. Another study by Guo et al. (2019) reported production of a poly (1,8-octanediol citrate) (POC)/poly(ε-caprolactone) (PCL) composite elastomer via an in-situ copolymerization blending technique. A completely transparent elastomer was produced as a result of partial incorporation of PCL into POC networks. The incorporation of the PCL into the process showed a positive result where the thermosetting time was decreased, thus producing a nonsticky elastomer.

1.18.2

GRAFTING

Polymer grafting is a technique that involves the covalent attachment of monomers to a polymer chain(Koshy et al., 2016). This method encompasses two distinct approaches: grafting-to and grafting-from. Grafting-to involves the attachment of an end-functionalized polymer chain to a solid substrate, whereas grafting-from entails initiating the grafting reaction through surface polymerization (Minko, 2008). An example of biopolymer grafting is in the surface modifcation of biodegradable electrospun nanofber scaffolds conducted by Park et al. (2007). Electrospun nanofber scaffolds are widely used in tissue engineering to deliver transplanted cells to target areas and provide mechanical support from physiological loads. They provide a good microenvironment for cell adhesion, proliferation and differentiation due to their properties such as tunable porosity, high surface-to-volume ratio and ease of surface functionalization (Mo et  al., 2018). To make a biodegradable electrospun nanofber scaffold, a synthetic biopolymer was used consisting of PGA, PLLA and PLA. The method used was grafting-to in situ polymerization of acrylic acid (AA). This method added a carboxylic functional group to the scaffold, making it hydrophilic and cell compatible. The result of this surface modifcation through the grafting method signifcantly improved fbroblast adhesion and proliferation on the surfacemodifed scaffold.

1.18.3

TRANSESTERIFICATION

Transesterifcation has been successfully used to produce polymers and blends with better properties. A study by Lee et al. (2001) discussed the transesterifcation reaction of barium sulfate (BaSO4) on a poly(butylene terephthalate)/poly(ethylene terephthalate) blend system. In this study, a titanate coupling agent (TCA), which is organic particles, was introduced into BaSO4 (inorganic particle), which acted as a surface modifer of BaSO4 in order to improve the interfacial adhesion inorganic fller and organic polymer matrix. When PBT and PET are blended, a block copolymer is formed from the mixture. It is diffcult to separate the block copolymer state through transesterifcation; thus, the end result is a random copolymer with a decrease in molecular weight. It has been concluded that BaSO4 suppresses the

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transesterifcation reaction during the melt blending process, as the melting endotherm peak decreases with an increase in PBT concentration. The result from the experiment shows that the melt blend of PBT/PET with no BaSO4 showed outstanding transesterifcation reactions with a decrease in mechanical properties compared to PBT/PET with surface modifcation BaSO4 or modifed BaSO4, which showed a slight transesterifcation reaction but an increase in mechanical properties (fexural strength). Thus, the surface modifcation was shown to improve the polymer blend properties.

1.18.4 REACTIVE COUPLING AGENT The use of reactive coupling agents is also one of the ways to improve polymer properties and blends. Jariyasakoolroj and Chirachanchai (2014) proposed a study on a polymer blend of PLA with silane modifed starch. Silane is one of coupling agents that function to modify starch via the hydrolyzable and organofunctional reactive sides in order to form covalent bond with PLA. A series of trimethoxy silane coupling agents such as 3-glycidoxypropyl trimethoxysilane (GPMS), 3-aminopropyl trimethoxy silane (APMS) and 3-chloropropyl trimethoxysilane (CPMS) were used to coupleg with starch, which then formed GP-starch, AP-starch and CP-starch, respectively. The surface modifcation of starch by organofunctional silane coupling agents has successfully been traced based on the FTIR results obtained. Based on the result, it was stated that the coupling of silane on starch might happen via silanol linkages. The nuclear magnetic resonance (NMR) method used to study the structure of starch modifed by silane agents gave information about bond formation between the components. Compared to other silane modifed starch, only CP-starch formed covalent bonds with PLA during blending, thus providing compatibility between PLA and starch. The modifed starch (CP-starch) also played a role as a nucleating agent, where it accelerated the crystallization rate in PLA, and this was proved by the increasing rate in degree of crystallinity of the PLA/CP-starch blend. The mechanical properties of tensile strength and elongation at breaks of the flm produced from the reactive blend also increased compared to pure PLA and other PLA/ silane-modifed starch flms.

1.19

CONCLUSIONS

From the classifcation and chemistry of polymers, it can be concluded that there are variety of polymers being produced either biologically or synthetically via chemical modifcation. Nevertheless, the preference for polymers used in bio-plastics always emphasizes two characteristics, biodegradability and biobased origins. The main motivation behind the use of either biobased polymers or biodegradable polymers is always to reduce the carbon footprint, greenhouse emissions, waste pollution and exploitation of fossil fuel. Apart from that, the chemical characteristics of each polymer have been studied extensively, as they assist in new or novel material production. This can be seen from the examples given for biopolymer blends and bionanocomposites whereby two or more polymers can be mixed or incorporated with fllers to achieve better performance for targeted usage.

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Singh, R. S., & Kaur, N. (2019). Understanding response surface optimization of medium composition for pullulan production from de-oiled rice bran by Aureobasidium pullulans. Food Science and Biotechnology, 28(5). https://doi.org/10.1007/s10068-019-00585-w Sionkowska, A. (2003). Interaction of collagen and poly(vinyl pyrrolidone) in blends. European Polymer Journal, 39(11), 2135–2140. https://doi.org/10.1016/S0014-3057(03)00161-7 Sionkowska, A. (2011). Current research on the blends of natural and synthetic polymers as new biomaterials: Review. Progress in Polymer Science (Oxford), 36(9), 1254–1276. https://doi.org/10.1016/j.progpolymsci.2011.05.003 Sionkowska, A. (2015). The potential of polymers from natural sources as components of the blends for biomedical and cosmetic applications. Pure and Applied Chemistry, 87(11– 12), 1075–1084. https://doi.org/10.1515/pac-2015-0105 Sionkowska, A., Kaczmarek, B., Lewandowska, K., Grabska, S., Pokrywczyńska, M., Kloskowski, T., & Drewa, T. (2016). 3D composites based on the blends of chitosan and collagen with the addition of hyaluronic acid. International Journal of Biological Macromolecules, 89. https://doi.org/10.1016/j.ijbiomac.2016.04.085 Sionkowska, A., Skopinska-Wisniewska, J., & Wisniewski, M. (2009). Collagen-synthetic polymer interactions in solution and in thin flms. Journal of Molecular Liquids, 145(3), 135–138. https://doi.org/10.1016/j.molliq.2008.06.005 Siracusa, V., & Blanco, I. (2020). Bio-polyethylene (Bio-PE), Bio-polypropylene (Bio-PP) and Bio-poly(ethylene terephthalate) (Bio-PET): Recent developments in bio-based polymers analogous to petroleum-derived ones for packaging and engineering applications. Polymers, 12(8). https://doi.org/10.3390/APP10155029 Sivaram, S. (2017). Giulio Natta and the origins of stereoregular polymers. Resonance, 22(11). https://doi.org/10.1007/s12045-017-0568-9 Sonseca, A., Madani, S., Muñoz-Bonilla, A., Fernández-García, M., Peponi, L., Leonés, A., Rodríguez, G., Echeverría, C., & López, D. (2020). Biodegradable and antimicrobial pla—ola blends containing chitosan-mediated silver nanoparticles with shape memory properties for potential medical applications. Nanomaterials, 10(6). https://doi. org/10.3390/nano10061065 Souza, P. M. S., Morales, A. R., Marin-Morales, M. A., & Mei, L. H. I. (2013). PLA and montmorilonite nanocomposites: Properties, biodegradation and potential toxicity. Journal of Polymers and the Environment, 21(3). https://doi.org/10.1007/s10924-013-0577-z Sperling, L. H. (2000). History and development of polymer blends and ipns. In Applied polymer science: 21st century (Issue 2, pp.  343–354). American Chemical Society Division of Polymeric Materials: Science and Engineering. https://doi.org/10.1016/ b978-008043417-9/50020-9 Subramanian, P. M. (1985). Permeability barriers by controlled morphology of polymer blends. Polymer Engineering & Science, 25(8). https://doi.org/10.1002/pen.760250810 Surendran, A., Lakshmanan, M., Chee, J. Y., Sulaiman, A. M., Thuoc, D. Van, & Sudesh, K. (2020). Can polyhydroxyalkanoates be produced effciently from waste plant and animal oils? Frontiers in Bioengineering and Biotechnology, 8. https://doi.org/10.3389/ fbioe.2020.00169 Suter, U. W. (2013). Why was the macromolecular hypothesis such a big deal? Advances in Polymer Science, 261. https://doi.org/10.1007/12_2013_251 Sworn, G. (2021). Xanthan gum. In Handbook of hydrocolloids (pp.  833–853). Elsevier. https://doi.org/10.1016/B978-0-12-820104-6.00004-8 Syuhada, N., Yazid, M., Abdullah, N., Muhammad, N., & Matias-Peralta, H. M. (2018). Application of starch and starch-based products in food industry. Journal of Science and Technology, 10(2), 144–174. https://doi.org/10.30880/jst.2018.10.02.023

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Szabo, K., Teleky, B. E., Mitrea, L., Călinoiu, L. F., Martău, G. A., Simon, E., Varvara, R. A., & Vodnar, D. C. (2020). Active packaging-poly (vinyl alcohol) flms enriched with tomato by-products extract. Coatings, 10(2). https://doi.org/10.3390/coatings10020141 Takahashi, S., Okada, H., Nobukawa, S., & Yamaguchi, M. (2012). Optical properties of polymer blends composed of poly(methyl methacrylate) and ethylene-vinyl acetate copolymer. European Polymer Journal, 48(5), 974–980. https://doi.org/10.1016/j. eurpolymj.2012.02.009 Takayama, T., & Todo, M. (2006). Improvement of impact fracture properties of PLA/PCL polymer blend due to LTI addition. Journal of Materials Science, 41(15), 4989–4992. https://doi.org/10.1007/s10853-006-0137-1 Talón, E., Trifkovic, K. T., Nedovic, V. A., Bugarski, B. M., Vargas, M., Chiralt, A., & González-Martínez, C. (2017). Antioxidant edible flms based on chitosan and starch containing polyphenols from thyme extracts. Carbohydrate Polymers, 157, 1153–1161. https://doi.org/10.1016/j.carbpol.2016.10.080 Tănase, E. E., Popa, M. E., Râpă, M., & Popa, O. (2015). Preparation and characterization of biopolymer blends based on polyvinyl alcohol and starch. Romanian Biotechnological Letters, 20(2), 10306–10315. Tang, X. Z., Kumar, P., Alavi, S., & Sandeep, K. P. (2012). Recent advances in biopolymers and biopolymer-based nanocomposites for food packaging materials. Critical Reviews in Food Science and Nutrition, 52(5), 426–442. https://doi.org/10.1080/10408398. 2010.500508 Tarkanian, M. J., & Hosler, D. (2011). America’s frst polymer scientists: Rubber processing, use and transport in mesoamerica. Latin American Antiquity, 22(4). https://doi. org/10.7183/1045-6635.22.4.469 Thakkar, R., Thakkar, R., Pillai, A., Ashour, E. A., & Repka, M. A. (2020). Systematic screening of pharmaceutical polymers for hot melt extrusion processing: A comprehensive review. International Journal of Pharmaceutics, 576. https://doi.org/10.1016/j. ijpharm.2019.118989 Thomas, S., Durand, D., Chassenieux, C., & Jyotishkumar, P. (2013). Handbook of biopolymer-based materials: From blends and composites to gels and complex networks. In Handbook of biopolymer-based materials: From blends and composites to gels and complex networks. https://doi.org/10.1002/9783527652457 Tian, H., Guo, G., Fu, X., Yao, Y., Yuan, L., & Xiang, A. (2018). Fabrication, properties and applications of soy-protein-based materials: A review. International Journal of Biological Macromolecules, 120. https://doi.org/10.1016/j.ijbiomac.2018.08.110 Tian, H., Tang, Z., Zhuang, X., Chen, X., & Jing, X. (2012). Biodegradable synthetic polymers: Preparation, functionalization and biomedical application. Progress in Polymer Science (Oxford), 37(2), 237–280. https://doi.org/10.1016/j.progpolymsci. 2011.06.004 Tiimob, B. J., Rangari, V. K., Mwinyelle, G., Abdela, W., Evans, P. G., Abbott, N., Samuel, T., & Jeelani, S. (2018). Tough aliphatic-aromatic copolyester and chicken egg white fexible biopolymer blend with bacteriostatic effects. Food Packaging and Shelf Life, 15. https://doi.org/10.1016/j.fpsl.2018.01.001 Tokiwa, Y., Calabia, B. P., Ugwu, C. U., & Aiba, S. (2009). Biodegradability of plastics. International Journal of Molecular Sciences, 10(9), 3722–3742. https://doi.org/10.3390/ ijms10093722 Torres, F. G., Troncoso, O. P., Pisani, A., Gatto, F., & Bardi, G. (2019). Natural polysaccharide nanomaterials: An overview of their immunological properties. International Journal of Molecular Sciences, 20(20). https://doi.org/10.3390/ijms20205092

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Biodegradation and Compostable Biopolymers

2.1 BIODEGRADATION AND COMPOSTABLES IN GENERAL Plastic flm is made from a petroleum-based material that is nonbiodegradable due to the fact that it is made from oil. Thompson et al. (2009) stated that around 4% of global oil supply is used as feedstock for the production of plastics, with a comparable amount of energy consumed in the process. Plastics make up only 10% of all recycled garbage, but they make up a signifcantly larger proportion of the litter that accumulates along shorelines (Barnes et al., 2009). It could take years for non-biodegradable materials to degrade. As a result, it has become a problem that will pollute our environment and fll landflls with non-biodegradable waste. Nonbiodegradable materials can clog streams, destroy marine organisms, and pollute a river and its surrounding areas. A broad variety of oil-based polymers are currently used in packaging applications, according to a journal report by (Kamarudin et al. 2022). They are almost all non-biodegradable, and some of them are impossible to recycle or reuse since they are complicated composites in which the contamination levels are different. Plastic bags create a lot of waste, and they are the most widely used material in people’s lives. They are mostly made of polyethylene (PE), which is not biodegradable and takes a long time to break down. As a result, a large amount of garbage ends up in landflls. As the planet moves toward a more environmentally sustainable economy, many scientists have worked to create materials that are more environmentally friendly. Nevertheless, it is not an easy move because it necessitates a lengthy procedure and high manufacturing costs. According to environmental issues emerging in recent years, biodegradable polymers are the latest manufacturing components to be used as raw materials in manufacturing biobased materials. Biodegradable polymers can be prepared through a few methods such as modifcation, chemical synthesis, microbiological synthesis, enzymatic synthesis, and chemo-enzymatic synthesis (Zeng et al., 2016). The use of biodegradable polymers will help to ensure sustainability and reduce the environmental impact of oil-based polymer disposal (Abdul Khalil, 2023). Many people want to lead the way in reducing the negative impact on the environment. With the advent of bioplastic or biodegradable polymers as a replacement for oil-based or non-biodegradable polymers, it is possible to reduce the negative impact on the environment while still serving as a blueprint for other industries to manufacture environmentally friendly materials. Biodegradable polymers can withstand deterioration during their usage while also being biodegradable at the end of their useful life (Surya et al. 2022). A biodegradable DOI: 10.1201/9781003416043-2

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polymer, according to Leja and Lewandowicz (2010), is a polymer that degrades by microorganism metabolism. As a result, a biodegradable polymer can break down into carbon dioxide, water, and biomass (Wittaya, 2009). The conversion process of biopolymers into gases is called mineralization. When all biodegradable products or biomass have been absorbed and all carbon has been converted to carbon dioxide, mineralization is complete. Activated sludges, for instance, are used to process sewage fows in a wastewater treatment system in order to biotransform the organic compounds to complete their mineralization (Poznyak et al., 2019). Since the carbon dioxide emitted is already part of the biological carbon cycle, it does not lead to an increase in greenhouse gases (Song et al., 2009). According to Imre and Pukánszky (2013), such polymers are degradable if their end products result in a decrease in molecular weight compounds due to chain scission in the backbone, and degradation is completed. Biodegradable polymers can be made from a variety of sources, including wood and microorganisms (Vieira et al., 2011). Degradation is one of the disposal methods for polymers. Degradation is a type of decomposition that comes to a halt when polymers are fragmented by heat, moisture, sunlight, or enzymes, resulting in weakening of the polymers’ chains (Mohee et al., 2008; Song et al., 2009; Rujnić-Sokele & Pilipović, 2017; Ryan et al., 2018). Meanwhile, biodegradation is a term used to describe specifcally engineered biodegradable polymers that undergo complete mineralization (Kyrikou & Briassoulis, 2007). As the number of biopolymers generated has grown, so has interest in polymer biodegradation. The most challenging aspect of making biodegradable polymers is refning their chemical, physical, and mechanical properties, as well as their biodegradability (Leja & Lewandowicz, 2010). Many polymers classifed as “biodegradable” are simply “bioerodable”, “photodegradable”, or just partly biodegradable. Biodegradation is a form of decomposition that involves biological activity (Zeng et  al., 2016). In the same vein, Azevedo et  al. (2008) stated that the progressive destruction of a substrate mediated by complex biological activity is also referred to as biodegradation. Meanwhile, according to terminology used in environmental engineering, degradation refers to the deterioration or breakdown of materials that happens when microorganisms use an organic substrate as a source of carbon and energy (Poznyak et al., 2019). For most chemicals released into the atmosphere, biodegradation is expected to be the primary process of destruction. The degradation and assimilation of polymers by living microorganisms to create degradation products is via this mechanism. The microorganisms that cause biodegradation are bacteria and fungi (Gautam et al., 2007), which include yeasts and molds. Biodegradation is known to occur in three processes: biodeterioration, biofragmentation, and assimilation, without neglecting the abiotic infuences where the process can stop at each stage (Lucas et al., 2008). Biodeterioration is when microbial communities, other decomposer organisms, and abiotic factors all work together to break down biodegradable materials into tiny fractions. Deterioration is a form of surface deterioration that changes a material’s mechanical, physical, and chemical properties (Hueck, 2001; Walsh, 2001). The composition and properties of polymer materials infuence microbial growth. Environmental conditions such as humidity, temperature, and emissions in the atmosphere are also signifcant factors to consider (Lugauskas et al., 2003). Microbial deterioration can occur in several ways: physical

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by adhering to material surfaces; chemical through the development of microorganisms into materials; and enzymatic, which involves enzymes such as lipases, ureases, and proteases. Also, biofragmentation is a lytic process where the bonds within polymers are cleaved or broken down, thus generating oligomers and monomers (Lugauskas et al., 2003). The last stage of biodegradation is assimilation, which is described as a mechanism of microbial cells integrating atoms from particles of polymeric materials (A. Glaser, 2019). Biodegradation can occur either in aerobic or anaerobic conditions (Rizzarelli & Carroccio, 2014). Under aerobic conditions, where oxygen is present and carbon dioxide is produced, and in anaerobic conditions, where nitrate, sulfate, or another compound is present and methane is produced, biodegradation may take place (Grima et al., 2000; Kyrikou & Briassoulis, 2007; Poznyak et al., 2019). Aerobic biodegradation is defned as the destruction of microbial materials by microorganisms upon the presence of oxygen as fnal electron acceptor. In anaerobic biodegradation systems, degradation occurs when the compound is fully consumed by microorganisms, resulting in methane emissions. As mentioned, oxygen is the electron acceptor in aerobic metabolism. Microbial species readily adapt and attain high densities if biodegradation follows this trend of metabolism. As a consequence, the rate of biodegradation is easily restricted by oxygen availability rather than the intrinsic microbial capacity to degrade the polymer or other contaminants. In aerobic conditions, biodegradation is measured by the percentage of carbon content converted to carbon dioxide. In the interval, anaerobic biodegradation, which is in the absence of oxygen, uses nitrate or sulfate as the last electron acceptor. The rate of degradation under this system is limited by the reaction rate of active microorganisms, which results in slow adaption and thus requires months or years to degrade. Some biopolymers are biodegradable, and some are compostable. In this part, the composting of biopolymers will be discussed. According to Rudnik (2008), compostable polymers were frst introduced in the 1980s. In short, compostable biopolymers are polymers that undergo a composting process and will break down by 90% within six months or less. To be a compostable polymer, it should meet the following criteria: no visually distinguishable polymer traces and no ecotoxicity, which means there is no toxic material produced during the composting process (Sadeghi & Mahsa, 2015). Natural organic materials made from starches produced from rice, potato, tapioca, or other plants and vegetable matter have recently been mixed with biodegradable polymers to produce compostable products that can further reduce the environmental effect of carbon footprints. Rudnik (2008) states that compostable polymers can be prepared via blending methods, biotechnological techniques such as fermentation and extraction, and also through polymerization. The leading method is the blending procedure, as this method does not require much cost and energy. Starch-based polymeric blends are some of the extensively compostable biopolymer blends that are commonly being researched. For instance, Novamont, which is an innovation company, prepared a few polymeric blends based on starch by blending the starch with other biopolymers in the presence of a plasticizer or water by using an extruder (Bastioli, 1998). Meanwhile, an example of a compostable biopolymer produced via the biotechnological route is poly(hydroxybutyrate-co-hydroxy valerate) (PHBV), and one from polymerization is polylactic acid (PLA). Biocompostable

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materials provide an environmentally friendly alternative to other viable options and can help minimize social and economic inequality, reduce the effect of our consumption on the atmosphere, and create possibilities for making a cleaner and more prosperous world (Sadeghi & Mahsa, 2015). To recap, a material must meet the following conditions to be labelled “compostable”: mineralization process, disintegration into a composting device, and completion of biodegradation during composting system end use. Compostable biopolymers may be classifed based on their origin or method of preparation. Compostable biopolymers are made from renewable and petrochemical materials (Sadeghi & Mahsa, 2015). Compostable biopolymers from renewable resources are cellulose, chitosan, proteins, PLA, and poly(3-hydroxybutyrate) (PHB). Plants and microorganisms both generate cellulose, which is a ubiquitous and plentiful biopolymer (Mohamad Haafz et al., 2013). Many agricultural by-products, such as sugarcane, sorghum bagasse, corn stalks, rye, wheat, oats, and rice straws, can be used to make cellulose pulp (Rudnik, 2008). On the other hands, chitosan is the most common polysaccharide after cellulose. Chitosan is a high molecular weight biopolymer and a deacetylated derivative of chitin that can be found in marine crustacean shells and fungus cell walls (Di Martino et al., 2005). The next compostable biopolymer is proteins, which are random copolymers of amino acids (Rudnik, 2008). Protein also can be classifed based on its origin, plants and animals. Soy and potato are examples of protein from plants, while collagen and silk are examples from animal origin. Moreover, PLA is the only renewable resource thermoplastic polymer manufactured at a wide scale, over 140,000 tons per year among the available biopolymers (Mukherjee & Kao, 2011). PLA has been used widely in preparation of biopolymer blends and also in the development of biopolymeric materials. Next is PHB, which is a type of polyhydroxyalkanoate (PHA) (Bonartsev et al., 2011). PHB is synthesized by cells under growth-limiting conditions, which occur where the carbon supply is abundant but nitrogen, phosphorus, magnesium, oxygen, or sulfur is in a limiting concentration (Vishnuvardhan Reddy et al., 2009).

2.2

METHODS OF BIODEGRADATION AND COMPOSTING OF BIOPOLYMERS

There are several different methods of biodegradation that are mostly used in order to determine the biodegradation of biopolymer products: soil burial (Xu & Hanna, 2005; Parvin et al., 2010; Mittal et al., 2016; Rapisarda et al., 2019) and microbiological (Benedict et al., 1983) and enzymatic methods (Nobes et al., 1998). Because of its resemblance to real waste disposal conditions, soil burial is a common and normal procedure for the biodegradation of biopolymers. Azahari et al. (2011) worked on biodegradation of polyvinyl alcohol (PVOH)/corn starch (CS) blend flms by burying the flms in soil and compost. The biodegradation was determined by enzymatic degradation rate, weight loss of samples in the soil burial test, and tensile strength of the flms. The results for enzymatic degradation showed that the degradation rate of flms increases with an increase in starch content. This was due to the presence of excess hydroxyl groups that absorbed more enzymatic solution. A higher weight loss was observed in blended flm with a high content of corn starch compared to pure

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PVOH flm. This is attributed to the high biodegradability of corn starch compared to PVOH. For the tensile properties, the result showed a decreasing trend in tensile strength and elongation at break when the corn starch content in the flms increased, which is due to the amorphous nature of starch. According to this report, the increase in starch content increased the degradation of biopolymer flms. Microorganisms are involved in the deterioration of both synthetic and natural polymers (Gu, 2003). Macromolecular chains are dissolved in the presence of microorganisms such as bacteria and fungi, which initiates the biodegradation process. The molecular concentrations, molecular weights, and the presence of specifc microorganisms on the surfaces of polymer materials all infuence microbial degradation. Since polymer-degrading microorganisms are also responsible for the degradation of polymeric products, many researchers have done studies based on microbial degradation. Ruiz-Dueñas and Martínez (2009) studied the microbial degradation of lignin in nature. Further, the molecular architecture of the lignin polymer, in which various non-phenolic phenylpropanoid units form a complex three-dimensional network connected by a variety of ether and carbon–carbon bonds, makes it highly resistant to chemical and biological degradation. Extracellular haemperoxidases working synergistically with peroxide-generating oxidases have formed a novel technique for lignin degradation based on unspecifc one-electron oxidation of the benzenic rings in the different lignin substructures by ligninolytic microbes. Similar reports based on microbial biodegradation of biopolymer and biopolymer blends can also be found in other works (Fields et al., 1974; Benedict et al., 1983; Siracusa, 2019). The growing use of polymers in industry has resulted in the environmentally sensitive problem of waste disposal. Enzymes play an important part in polymer biodegradation (Banerjee et  al., 2014). Tokiwa and Calabia (2006) explained the biodegradation of polylactide (PLA) in their study. The hydrolysis of aliphatic polyesters is a two-step enzymatic degradation process. The frst step is for the enzyme to bind to the substrate’s surface through its surface-binding domain, and the second step is for the ester bond to be hydrolyzed. Kikkawa et  al. (2002) performed a study on enzymatic degradation on poly(L-lactide) (PLLA) flm. In their study, it was concluded that the rate of degradation in the free amorphous region of the flm was faster than in the restricted amorphous region. Other literature has also discussed the role of enzymes in biodegradation of biopolymers (Nakamura et al., 2001) and biopolymer mixtures (Ishigaki et al., 1999; Vikman et al., 1999; Spiridon et  al., 2008). Tsuji and Ishizaka (2001) used enzyme to study the mechanisms of PLLA/poly(ε-caprolactone) (PCL) blend degradation. They prepared porous biodegradable PCL flms by removing PLLA from blended flms using proteinase K as a hydrolysis enzyme for PLLA. The results showed that the enzymatic hydrolysis rate of blended flms increased, which is suggested to be due to the hydrolysis of PLLA that is catalyzed by proteinase K at interfaces of PLLA- and PCL-rich phases and also at the flm surfaces. Composting is an option for waste treatment where material undergoes recovery and produces a useful product (Song et al., 2009). Based on the defnition from ASTM D6400, composting is defned as “a managed process that controls the biological decomposition and transformation of biodegradable materials into a humus-like substance called compost” (Rudnik, 2008). In other words, the composting process

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has the capability to convert biodegradable materials into valuable soil amendment materials (compost). Compost is a fexible commodity made from the composting and biodegradation of organic waste that is generated in commercial or domestic environments (Sadeghi & Mahsa, 2015). Compost is an organic matter that is rich in nutrients, which can improve soil quality, help plant development, enhance waterholding ability, and minimize the use of chemical fertilizers. Compost also has other advantages such as acting as a soil conditioner, organic fertilizer, and natural pesticide. Adding compost to soil can improve the soil structure by reducing bulk density and improve porosity, which then results in better plant growth. Also, adding compost to soil can modify and stabilize the soil pH, depending on the soil and compost pH. Furthermore, another capability of compost is to control soil erosion to reduce topsoil loss. As mentioned, compost is capable of holding water; thus it can remain on the soil surface to reduce the amount of rain that falls onto the soil. Organic matter is broken down by microorganisms such as bacteria and fungi during the composting process, resulting in the production of carbon dioxide, heat, water, and compost. There are three type of composting methods: in-vessel methods, aerated static pile methods, and windrow methods (Sadeghi & Mahsa, 2015). For the in-vessel method, signifcant volumes of waste may be processed in a small amount of space compared to other composting methods (Rudnik, 2019). Also, in this method, the organic material is composted in a silo, drum, concrete-lined trench, batch tub, or other similar equipment. The environmental conditions are well controlled and regulated, and the substance is aerated by being mechanically mixed and agitated. For instance, Ghorpade et  al. (2001) conducted a composting process in at laboratory scale by using extruded PLA sheets combined with yard waste. This experiment was to examine the composting capability of PLA in combination with garden waste. In the experiment, the mixture of PLA and garden waste was mixed in a composting vessel and stayed for four weeks. The result showed that the yard waste compost/PLA mixture with 30% PLA concentration lowered the compost pH. Moreover, it was also observed that the PLA molecular weight was reduced as an effect of composting. The study demonstrated that the addition of PLA concentrations below 30% to yard waste compost is an effcient composting method. In the aerated static pile method, organic waste is mixed into a large pile and then aerated by drawing air out of the pile or allowing air to fow through the pile (Rudnik, 2019). According to the U.S. Environmental Protection Agency (EPA), composting in an aerated static pile generates compost very easily within three to six months. This method can handle a largely homogeneous blend of organic waste and compostable municipal solid waste. However, the aerated static pile method is ineffective for composting animal waste or grease from the food processing industry. In a study reported by Colón et al. (2013), the composting process involving compostable diapers combined with the organic fraction of municipal solid waste shows the mixture underwent composting in a static, forced-aerated composting reactor over a period of 41 days. It was reported that the composting process and the compost produced was not affected by the presence of compostable diapers, as no pathogenic microorganisms were found during the full-scale composting experiment. The study’s signifcant point is that the composting of an organic fraction of municipal

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solid waste with compostable diapers may be a novel way to transform this waste into high-quality compost. The windrow composting method involves forming compostable materials into rows of long piles known as windrows (Sadeghi & Mahsa, 2015). In this method, the materials are aerated by turning the pile either by manually or mechanically on a daily basis. This pile will produce enough heat to keep temperatures stable. It is small enough to cause oxygen to fow to the center of the windrow. The composted component is ready for assembly and shipment to end users when it has reached the desired stage of decomposition (Arvanitoyannis, 2013). Itävaara et  al. (1997) proposed a study on biodegradable polymer windrow composting. It was observed that several environmental factors infuence the degradation of biodegradable samples, including temperature, pH, oxygen concentration, and water content. The experiment also did a trial on plant growth in which they used barley and radish seed to see the effect of degradation products from biodegradable samples on them. The result showed that the compost made from a polymer sample did not show any toxicity in plant growth.

2.3

BIODEGRADABILITY OF BIOPOLYMERS

Biodegradable biopolymers are biopolymers that can be degraded by naturally occurring microorganisms such as bacteria, fungi, and algae into natural elements (biomass), carbon dioxide, and water (Wojtowicz, 2010). Biodegradable biopolymers are very eco-friendly because they are made from sustainable natural resources, and in the end, they will degrade into material that is harmless to the environment. Using biodegradable biopolymers conserves nature and produces less pollution; thus, it will make the earth move toward a healthy environment. Many researchers nowadays are focused on producing biodegradable biopolymers to replace conventional nonbiodegradable polymers. Undoubtedly, non-biodegradable polymers have advantages in term of technical properties and low cost, making them the most favorable alternative for many industries such as food packaging and biomedicine (Bassas-Galià, 2017; Babooram, 2020). For example, polystyrene was commercialized in large volumes for food packaging in the late 1940s (Robertson, 2019). Although it has many advantages, using non-biodegradable synthetic polymers for food packaging is not a wise choice because they are made from petroleum-based resources, which are nonrenewable energy and will take a thousand years to degrade, which can cause trouble in the current ecosystem (Kumaran et al., 2020). In biomedicine, non-biodegradable scaffolds were used in many applications such as defect repair, tissue healing, and cell transplantation. Using non-biodegradable scaffolds required an extra operation to remove it compared to biodegradable scaffolds that can degrade without requiring any surgical removal (Stewart et al., 2018; Mo et al., 2018). As stated earlier, the use of biopolymers helps to maintain sustainability and to reduce waste, but not all biopolymers are biodegradable. Polythioester (PTE) is an example of a non-biodegradable biopolymer, which is contrary to the idea of all natural-based polymers being biodegradable (Steinbüchel, 2005). Although PTE is synthesized by microbial fermentation, PTE is non-biodegradable by microorganism (Elbanna et al., 2004). Besides PTE, other biobased polymers such as bio-polyethene

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and bio-polypropylene are also non-biodegradable biopolymers, and these polymers are widely used in bioplastic applications (Andreeßen & Steinbüchel, 2019). Some bioplastics need to be non-biodegradable to produce more durable material. Nonbiodegradable bioplastics might not reduce waste, but manufacturing using biobased polymers can surely reduce greenhouse gas (GHG) emission and depletion of fossil resources (Allison & Bassett, 2015). Polymer are theoretically biodegradable, which means that they can be degraded by microbes under the right conditions, but many degrade at such slow rates that they are classifed as nonbiodegradable (Patel et al., 2011). According to the EN 13432 standard, a material or product is biodegradable if it can undergo a specifc degradation process caused by biological activity and can be measured by a standardized test method under specifc environmental conditions within a specifed period (Wojtowicz, 2010). In other words, a biodegradable polymer must fully decompose and break down into natural elements within a short period of time after disposal. The assessment of biodegradability of biopolymers can differ depending on the following factors. The factors that infuence the degradation of polymers are dependent on the physiochemical structural and environmental conditions and microbial population where the polymers are exposed (Folino et al., 2020). The basic properties that cause the degradation and biodegradation of polymers are their physical and chemical structures. Biodegradation rates differ among biopolymers due to differences in the structural and physicochemical properties of their surfaces, which allow for stronger or weaker microorganism attack on the surface (Volova et  al., 2010; Bátori et  al., 2018). The physicochemical structure of biopolymers encompasses aspects such as molecular structure, polymer chain length, crystallinity, and polymer composition. In particular, the chain length of the biopolymer infuences its biodegradation, where longer polymer chains are generally more prone to degradation. However, the crystallinity of the polymer is also a signifcant biodegradation parameter, as the amorphous parts of the polymer are simpler to degrade than the crystalline parts (Massardier-Nageotte et al., 2006). In a study by Woolnough et al. (2008), it was stated that a higher degree of crystallinity leads to a slower rate of biodegradation, with the amorphous phase degrading frst. They mentioned that biopolymer poly(3-hydroxyoctanoate) (PHO), which is an example of MCL-PHA, has lower crystallinity than poly [(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] [P(HBco-HV)]. Thus, P(HB-co-HV) was found to degrade faster than PHB in fresh water as well as in soil. As mentioned, polymer formulation also affects the biodegradation of biopolymers. Since many microorganisms are needed to target the different functions of the polymer, the more complicated the formula, the less degradable it is. For instance, polymers with rings seem to be more resistant to degradation. Biopolymers have the potential to biodegrade in natural habitats, which is a benefcial property. One of the most important aspects of their creation is the need to prevent the accumulation of petroleum-based wastes, some of which are not biodegradable in the ecosystem, especially in the oceans. Biodegradability, as previously noted, is dependent on the chosen environment and can vary from one environment to another. Thus, the rate of biodegradability of biopolymers is affected by the environmental conditions, as well as the type of biopolymers used. Karamanlioglu et  al. (2017) did a review on environmental factors involved in PLA biopolymer

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degradation and also the degradation of PLA in various environments. Since biotic and abiotic infuences coexist in nature, environmental degradation refers to the whole degradation process of given contents. Humidity, temperature, and catalytic species (pH and the presence of enzymes or microorganisms) are examples of environmental conditions that affect biodegradation. For instance, PLA degrades more quickly in alkaline conditions because hydroxide ions catalyze the cleavage of ester groups during hydrolysis. As a result, a high concentration of hydroxide ions in alkaline media accelerates PLA degradation (Cam et al., 1995; Tsuji & Ikada, 1998). Different environments such as soil and compost also infuence the biodegradation of biopolymers. Karamanlioglu et al. (2017) reported that PLA degradation in soil is much slower than in compost medium, owing to the environmental conditions present in compost, which has higher moisture content and temperature range and thus encourages PLA hydrolysis and assimilation by thermophilic microorganisms. Ohkita and Lee (2006) did a study on biodegradability of PLA and PLA flms. In the study, they observed the biodegradation of pure PLA and PLA flm buried in soil. After six weeks, pure PLA showed only a little degradation. Meanwhile, PLA flm buried in soil for 120 days at 25℃ showed no degradation as measured by weight loss of the flm. Regardless, scanning electron microscopy (SEM) detected actinomycete threads on the PLA flm, but there were no signs of degradation on the PLA flm surface. Beside the physical and chemical structure and environmental conditions the biopolymers are exposed to, microbial communities are also one of the factors in biodegradation of biopolymers. Biodegradation is caused by more than 90 different microorganisms in diverse environments (Emadian et al., 2017; Thakur et al., 2018). For example, PHAs can be directly degraded by microorganisms such as bacteria and fungi (Jendrossek et al., 1996; Folino et al., 2020). Since water is an essential requirement for microorganism growth and reproduction, water and moisture may play an important role in the biodegradation of biopolymers (Trivedi et al., 2016). As moisture levels rise, so does microbial activity, and biopolymers biodegrade at a faster rate. Commonly, polymers with shorter chains, more amorphous parts, and simpler formulas are more vulnerable to microorganism biodegradation. The infuence of fungi and bacteria is one major cause, emphasizing the importance of microbial communities in biodegradation. Many studies have focused on role of bacteria and fungi in biopolymer biodegradation. For PHA compounds, a few bacteria such as Bacillus, Pseudomonas, and Streptomyces were reported as PHA-degrading microorganisms (Jendrossek et al., 1996). Itävaara et al. (2002) conducted a study on biodegradation of polylactides by soil bacteria in aerobic (aquatic) conditions. Polylactic acid degraded up to 90% after 55 to 60 days, and poly-L-lactic acid degraded up to 50% after 40 to 45 days, according to the researchers. Since such microorganisms have a small enzyme population in the absence of oxygen, oxidation rates are lower in anaerobic conditions (Thakur et al., 2018). On the other hand, Tritirachium album was the frst fungus to be discovered capable of degrading PLA in the scientifc literature (Jarerat & Tokiwa, 2001). Trivedi et al. (2016) did a review on the effect of microbes on biopolymers. It was stated that biopolymer PCL can be degraded by the fungi Penicillium and Aspergillus. In the review, it is mentioned that PCL can be degraded by Aspergillus strain ST-01, which was isolated from soil and incubated at 50°C for six days.

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COMPOSTING OF BIOPOLYMERS

Compostable biopolymers are similar to biodegradable biopolymers, with the same goal of reducing waste on earth. However, compostable biopolymers degrade at compost sites with specifc conditions. Compared to biodegradation, the composting process will yield excellent results where material completely degrades much faster into carbon dioxide, water, inorganic compounds, and biomass with no distinguishable or toxic residue (Kale et al., 2007). The outcome of the composting process can be utilized as bio-fertilizer to replace chemical fertilizer. Chemical fertilizer is responsible for greenhouse effects, environmental pollution, and the death of soil organisms and marine inhabitants (Ayilara et al., 2020). These results cause farmers to focusing on producing bio-fertilizers by either industrial composting and natural composting, also known as home composting or backyard composting. Generally, the industrial composting process is much faster compared to natural composting because biodegradation is optimized by controlling the conditions such as shredding and controlling the temperature and oxygen level. Natural composting can be done at home without the specifc conditions of industrial composting. This is because natural composting can compost material in the home environment, room temperature, and natural microbial community. Typically, natural composting consists of organic material such as food waste, wood, yard trimmings, and so on (Reyes-Torres et  al., 2018). With the emergence of biopolymer products, they also can be included as a substance for natural composting. However, not all biodegradable biopolymers are suitable to undergo natural composting due to factors such as taking long time to compost naturally at ambient temperatures (Song et al., 2009). It is suggested to compost only products with certifcation. Examples of certifcation bodies that offer a home compostable certifcation program are TÜV Austria (formerly known as AIB Vinçotte) and DIN Certco (Endres & Siebert-Raths, 2011). The process of natural composting can occur in a small pile or preferably in a composting bin. The composting process may take up to two years, but with turning, it can be reduced to three to six months. Turning usually done manually, or it can be done with the help of an advanced composting bin equipped with rotating drums (Kale et al., 2007). Ventilation is also important to ensure the composting material undergoes aerobic processes to produce a stable, non-toxic, pathogen-free, and plant nutrient–rich product (Ermolaev et al., 2014). The purpose of industrial composting is to convert biodegradable waste of biological origin into stable, sanitized products for agricultural use. It is commercial composting at a large scale with certain conditions to help increase the biodegradation rate (Kale et al., 2007). Some conditions that mean industrial composting can degrade faster at a large scale are high temperatures, around 50–70°C; high moisture; and the oxygen content in aerobic process. These controlled conditions allow increasing microbial activity, and the degradation process becomes faster (Endres & Siebert-Raths, 2011). Materials that can be use for industrial composting are the same as materials for natural composting, such as mechanical pulp, food waste, and home-compostable certifcate biopolymers. However, industrial composting can compost what natural composting cannot, for example, PLA, which takes a long time to degrade in natural composting (Rujnić-Sokele & Pilipović, 2017). Like home

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composting, industrial composting also has certifcation bodies that offer certifcation for industrial composting. Some example organizations are DIN Certco, TÜV Austria, and the Australasian Bioplastics Association (ABA). Those organizations refer to the EN 13432 standard, which is needed to pass tests related to the biodegradability, disintegrability, ecotoxicity, and quality of compost of the material (Briassoulis et al., 2010). There are three main phases in industrial composting: the mesophilic phase, then the thermophilic phase, and fnally the cooling or maturation phase (Palaniveloo et al., 2020).

2.5

ENVIRONMENTAL IMPACT OF BIOPOLYMERS

Synthetic polymers such as PP, PE, and HDPE are extensively employed in food packaging and food containers due to their durability, supply, and cost-effectiveness. However, the disposal of these packaging materials has become a solid waste management challenge and a signifcant contributor to environmental pollution (Kamarudin et al., 2022). Also, durability and the undesirable accumulation of synthetic polymers are the main threats to the environment through contaminating natural resources such as water quality and soil fertility (Pathak & Navneet, 2017). To overcome these drawbacks, biobased and biodegradable polymeric materials may be among the most appropriate alternatives for certain applications due to concerns about the global climate and the growing diffculties in handling solid wastes (Sudesh & Iwata, 2008). In recent years, the utilization of biopolymers has shown a marked increase in interest in a variety of industries such as packaging, agriculture, medicine, and others (Shamsuddin et al., 2018). Various biopolymers, biosynthetic polymers, chemosynthetic polymers, their blends, and composites have been studied for packaging applications. Commonly, biopolymers are mostly derived from renewable resources and can be classifed into three groups: polymers made from a renewable resource/ biomass, such as agro-polymers from agricultural resources; polymers from microbial products or animal origin, such as polyhydroxyalkanoates, which may be useful in medical and pharmaceutical practices; and chemically synthesized biodegradable polymers obtained from petrochemical resources or modifed from natural polymers, as presented in Figure 2.1 (Kumaran et al., 2020). Typical biopolymers such as polylactic acid and starches have made signifcant inroads in the packaging industry due to their advantages such as good strength and fexibility, non-toxicity, oxygen impermeability, strong moisture resistance, storage stability over a large temperature range, and low cost for both the raw materials and the processing technology. This shows the characteristics of biopolymers meet the criteria required for food packaging (Sudesh & Iwata, 2008; Buffum et al., 2015). The development of these biopolymers has several potential benefts over petroleum-based polymers, such as cost effectiveness, environmental friendliness, and user-friendly materials. Mostly, the uses of biodegradable materials are intended to lead towards long-term sustainability and a reduction in the environmental impacts of oil-based polymer disposal (Song et al., 2009). According to universal researchers, the production of biopolymers does not pose any signifcant threat to human beings, plants, and animals. Instead, biopolymers can lead to a safe and healthy environment

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FIGURE 2.1

Biopolymers and Biopolymer Blends

Structures of the three major sources of biopolymers depending upon origin.

for universal sustainable development (Shamsuddin et al., 2018). Furthermore, the impact created on the environment by biopolymers, especially for CO2 emissions, is lower compared to synthetic polymers. This is because they could create a “carbon neutral” life cycle, in which the net amount of carbon dioxide emitted into the atmosphere remains relatively constant (Sudesh & Iwata, 2008; Balart et al., 2020). However, the biggest obstacle to using biopolymers as a substitute for petroleumbased polymers is their thermal instability at low temperatures, as well as their low mechanical strength (Sadasivuni et al., 2020). Other than that, biopolymers have the ability to degrade within a short lifespan compared to synthetic polymers (Shamsuddin et  al., 2018). The most widely accepted ASTM definition of biodegradable polymers states that the degradation of these polymers occurs through the activity of naturally occurring microorganisms, such as bacteria, fungi, and algae. This natural decomposition process results in the production of products such as carbon dioxide (CO2) and water (Mohanty et al., 2000). In fact, bacterial and fungal organisms are the most common biological agents in nature, and they can degrade both natural and synthetic polymers in different ways (Pathak & Navneet, 2017). During the bioremediation process where microorganisms are employed to remediate contaminated environments, essential nutrients and certain chemicals are provided to support their growth and development. This enables these microorganisms to effectively eliminate pollutants present in contaminated areas (Nandal et al., 2015). Studies in the literature have reported the use of biopolymers such as polypeptides and polysaccharides as critical factors in the processes of bioremediation, recovery of polluted environments, and remediation of heavy metals and petroleum derivatives through natural biopolymers, since they present a high affinity

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with biological systems (Bedor et al., 2020). As a result, all attempts should be made to procure products from natural sources that have a high rate of biodegradation in the ecosystem, occupying positions and displacing conventional plastics in order to restore the environment that has been harmed so far by the indiscriminate use of synthetic polymers and prevent further deterioration (Kamarudin et al., 2022).

2.6 CONCLUSIONS Conventional synthetic polymers are challenging to recycle due to their highly heterogeneous nature, which leads to environmental pollution. The environmental impacts associated with plastic waste management techniques cannot be ignored. In contrast, bioplastics offer a viable solution to reduce dependence on conventional petrochemical-based synthetic plastics. Renewable resources like agricultural crop residues and woody biomasses can be used as raw materials to recover the building blocks of bioplastics and biopolymers. Furthermore, bioplastics are biocompatible and biodegradable and possess mechanical properties equal to or better than petrochemical-based plastics. Microbe-derived biopolymers show promise in various ways, but their production still needs to be cost effective. Currently, biodegradable packaging is used for food items that do not require high impermeability to oxygen and water vapor and have a short storage period, such as fresh green groceries and fruits, or for long-storage products like dumplings and fries that do not demand excessive impermeable properties. However, the range of available flms offers various properties that can be tailored for packaging other food products with stricter requirements. Recent advancements in biotechnology and material engineering have positively impacted the production of biopolymers and the understanding of their biodegradation characteristics. Further research and development in recycling techniques, large-scale production, cost reduction, durability, sustainability, greenhouse gas emissions, and optimized biodegradation are needed for bioplastics. Inedible plant residues, microbial biomass, and derivatives containing cellulose fbers and lignin biopolymers could be suitable alternatives for bioplastic manufacturing. Bioplastics are considered environmentally friendly products in terms of sustainability and environmental risk assessment. Therefore, government support and social awareness are crucial to facilitate a shift from petrochemical products to biobased products.

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3.1 BIONANOCOMPOSITES Before understanding why bionanocomposites play an important role in today’s research and industry felds, it is crucial to lay out the fundamental context of bionanocomposites. A bionanocomposite system is basically formed from a combination of two or more materials with different properties (Zabihi et al., 2018), as shown in Figure 3.1. The different properties of the material have the ability to complement each other, resulting in enhanced physical, mechanical, and thermal properties. Bionanocomposite is commonly used to describe nanocomposites that incorporate a biologically derived polymer (biopolymer) in conjunction with an inorganic component, with at least one dimension at the nanoscale to exhibit signifcant performance on a material. Back in the early 1990s, Toyota pioneered the use of nanocomposites. During their research, they made a breakthrough by demonstrating that montmorillonite (MMT) could be exfoliated into individual nanoparticles. This breakthrough substantially improved the dimensional stability, water resistance, and gas barrier properties of nylon-6. Consequently, this discovery sparked widespread interest and initiated numerous research endeavors in the feld of nanocomposites (Hussain et al., 2006). However, bionanocomposites made of biobased materials or biopolymers have been separated out to be an individual class from nanocomposites. This is due to the difference in preparation process, properties, and functionalities found between nanocomposites and bionanocomposites (M. H. Mousa et al., 2016). Bionanocomposites have gained tremendous attention in interdisciplinary areas, including biology, materials science, and nanotechnology. The curve of research is progressing, as shown in Chapter 1, due to the rise of environmental concerns. These materials are often referred to as natural nanocomposites, green nanocomposites, and biobased nanocomposites, owing to their reliance on constituents derived from biological sources (Sen, 2020). Jeevanandam et al. (2018) defned them as materials that contain a constituent or constituents of biological origin and nanoparticles with at least one dimension in the range of 1–100 nm. The biological origin refers to natural biopolymers such as polysaccharides, proteins, and natural gums derived from plants, algae, or animals (Jeevanandam et al., 2018). In the present day, numerous bionanocomposites have been meticulously developed through extensive research. Among the various bionanocomposites, which encompass flms, hydrogels, and aerogels, Table 3.1 provides a comprehensive overview of their respective advantages and disadvantages in comparison to conventional DOI: 10.1201/9781003416043-3

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FIGURE 3.1 Six main properties of bionanocomposites, which result in improvement of physical, mechanical, and thermal properties.

TABLE 3.1 Advantages and Disadvantages of Bionanocomposites over Nanocomposites Advantages • • • • • • • •

Biodegradable Biocompatible Renewable resources Abundant Reduced packaging volume, weight, and waste Some are edible Function as carriers for antimicrobial and antioxidant agents Microencapsulation and controlled release of active ingredients

Disadvantages • • • •

Lower mechanical strength Lower thermal stability Higher moisture absorption Preformed polymers

nanocomposites (Borgonovo & Apelian, 2011; Thakur et al., 2017; Gunputh & Le, 2020). Nonetheless, the enhancement properties of bionanocomposites depend on the characteristics of the biopolymers, the stoichiometric ratio of the constituent materials, the crosslinking among the constituent materials, and the biocompatibility between the biopolymer matrix and the reinforcement filler or particles. The interaction between fillers at nanometer-scale particles acts as a bridge in the biopolymer matrix (Cader Mhd Haniffa et al., 2016).

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3.2 CONSTITUENTS OF BIONANOCOMPOSITES Bionanocomposites are characterized by one or more discontinuous phase distributed in one continuous phase, whereby the continuous phase refers to the matrix, while the dispersed or discontinuous phase refers to the reinforcement particles or fillers (Siqueira et al., 2010; Abdul Khalil et al., 2019). The discontinuous phase has at least one dimension, roughly 10 −9 m (Figure 3.2). Examples of biopolymer matrices used to develop bionanocomposites can be from natural biopolymers that are fully biodegradable, biopolymers that are partially biodegradable thermoplastic polymers, thermoset polymers, or fully biodegradable petroleum-based biodegradable polymer matrices, as mentioned in Chapter 2. However, natural biopolymers and synthetic biodegradable polymers incorporated with nano-scale fillers/particles play a major role in the fabrication of bionanocomposites, as they determine the end result of the biodegradability properties (Othman, 2014). The matrix/continuous phase is made up of natural biopolymers, synthetic biodegradable polymers, or a combination of both materials. Each of these materials inherently possesses different characteristics, including molecular arrangement, active functional groups, bonding nature, thermal behavior, and solubility (George et  al., 2020; Zabihi et  al., 2018). The matrix phase is like the bulk material that contains particles or fillers dispersed in it. It acts like a glue cementing the particles or fillers together, as illustrated in Figure 3.3. Apart from the continuous phase, fillers play an equally important role to enhance the performance of the end product. This is because the fillers are able to change the characteristics of the matrix they are applied to and support the matrix by reducing shrinkage; increasing thermal tolerance; providing a reinforcement effect; increasing strength, especially tensile,

FIGURE 3.2

Phases of bionanocomposites between the dispersion of filler and matrix.

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and resistance to tears and compression; enhancing the exposure to solvents; and compounding cost reduction (Palem et al., 2018; Rajak et al., 2019). Fillers and reinforcements differ in terms of features and specifications. Fillers can be divided into two main groups, reinforcing fillers (e.g. carbon black, silica, and fibers) and inert fillers (e.g. clay, calcium carbonate, etc.) (Fahim et al., 2018; Gutiérrez et al., 2019; Gojayev et al., 2020). Carbon fiber reinforcement is a nonwoven, carbon fiber usually used to reduce cracking and extend life in bionanocomposites (Thakur et  al., 2017; Jeevanandam et  al., 2018). Different polymer reinforcement fillers/inert fillers are used to enhance the properties of the matrix. Some fillers can be targeted to improve specific properties such as brightness, density, abrasion, fineness, and oil absorption capability (Zare et al., 2017; Mishra et al., 2019), for instance, fiber reinforcement for concrete to enhance properties such as low shrinkage, good thermal expansion, substantial modulus of elasticity, high tensile strength, improved fatigue, and impact resistance (Müller et al., 2017; Fu et al., 2019; Abdul Khalil et  al., 2019). Among the many fillers, the common types of filler structures applied in bionanocomposites are spherical, rod-like, and layered structures, as shown in Figure 3.3. When designing bionanocomposites, there are many methods and techniques to synthesize bionanocomposites (Sanusi et al., 2021). Nevertheless, a few pointers

FIGURE 3.3 Common types of filler structures applied in bionanocomposites, including nanomaterials (NMs) with different dimensionalities. Reproduced from Poh et al. (2018).

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must be considered prior to fabrication to prevent unnecessary wastage and effort (Dantas de Oliveira and Augusto Gonçalves Beatrice, 2019; Dong et al., 2018; Rajak et al., 2019): • Determine the proportion of matrix and dispersed phase based on the intended use of the composite. • Reasons: Excessive fllers can give rigidity to the bionanocomposites. Thus, there is a balance to be struck that normally requires optimization. • Determine the size and shape of the fllers of the dispersed phase. • Reasons: This is because the size and shape of the particles/fllers can directly impact the reinforcement effect on the matrix. Usually, smaller particles provide more surface area for contact with the matrix, while longer fbers exhibit better reinforcement. • Ensure compatibility between the matrix and the particles. • Reasons: The interface between the matrix and fllers controls the overall performance of the bionanocomposite. The strength of a composite depends not only on the properties of the matrix but on how well it incorporates and interacts with the particles and fbers of the dispersed phase. Bionanocomposites have been applied in various industries, such as aerospace, food, biomedical, tissue engineering, paint, packaging, and glass coating (Thakur et  al., 2017). Nonetheless, they are especially useful for the biomedical industry and suitable to make scaffolds, implants, diagnostics, surgical instruments, and drug delivery systems because of their biocompatible and/or biodegradable properties whereby their degradation is mainly due to hydrolysis or mediated by metabolic processes (Song et al., 2018). They are also used in the cosmetics industry, with an emphasis on tissue engineering, drug and gene delivery, wound healing, and bio-imaging. They can also be used to replace some existing plastics, for example, low-density polyethylene (LDPE) and poly-vinylidene chloride (PVDC) (Wani, 2021; George et al., 2020).

3.3

TYPES OF BIONANOCOMPOSITES

In today’s research feld, the common bionanocomposites that have been studied extensively are the bionanocomposite flms, hydrogels, and aerogels. This section lays out the defnition and gives examples of bionanocomposite flms in the research feld and how are they fabricated at the lab or industrial scale. This section covers the fundamental and basic questions: 1. What are bionanocomposite flms, hydrogels, and aerogels? 2. What are examples of bionanocomposite flms, hydrogels, and aerogels and their functionality?

3.3.1

BIONANOCOMPOSITE FILMS

Bionanocomposites represent an evolving class of materials characterized by the presence of two or more distinct phases. They typically consist of a biopolymer, such

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as proteins, lipids, polysaccharides, or nucleic acids, serving as the continuous phase, and a fller, like silicates, carbon nanotubes (CNTs), or metal oxide acting as the discontinuous phase. These fllers possess at least one dimension within the nanometer range, typically ranging from 1 to 100 nanometers (Zubair and Ullah (2020). Since these bionanocomposite flms are made from natural materials, they are considered stable ecological materials with all of the advantages of both biopolymer and nanocomposite structures. Silicate and clay nanoplatelets, titanium dioxide, SiO2, carbon nanotubes, chitin, graphene, chitosan nanoparticles, cellulose-based nanofbers, starch nanocrystals, and other inorganics can be used as nanofllers for biopolymer matrices such as those as mentioned in Chapter 2 to enhance the performance of the entire composite, resulting in enhanced thermal, barrier, and mechanical characteristics (Sarfraz et al., 2021). Bionanocomposite flms should outperform their individual component materials in terms of optical, chemical, and mechanical properties. Depending on the preferred application, bionanocomposites can be customized to achieve certain physical properties such as transparency, oxygen permeability, and water permeability; mechanical properties such as tensile and elongation properties; thermal properties with regard to thermal stability; and biodegradable properties. The most common bionanocomposite flms are edible bionanocomposite flms and coatings and mulch bionanocomposite flms. The functions for each of these flms are further elaborated as follows (Unalan et al., 2014; Shankar & Rhim, 2018; Tuan Zainazor et al., 2020; Zubair & Ullah, 2020; Sarfraz et al., 2021): • Edible bionanocomposite flms and coatings • They are thin layers of an edible substance used in food application to preserve food products or a membrane of edible substance used in drugs. • They are divided into two types: monolayer and multilayer. • Monolayer flms: By altering the surface of other materials, a monolayer flm may be used in coating technology to enhance specifc properties. • Multilayer flms: They are made using a layer-by-layer formation technique. Hydrogen bonds and electronic interactions serve as driving forces between two layers deposited by solution dipping or spin coating in this technology. Non-electrostatic interactions, on the other hand, play a role as driving forces as two biobased materials are assembled layer by layer. • Since edible flms and coatings are promising non-toxic and non-polluting commodities that are safe for human use, they are more suitable for use as food packaging and drug delivery systems. • For food packaging, edible flms exhibit characteristics such as being fast to decompose and environmentally safe, as edible flm is made from renewable and biodegradable agriculture sources. Excellent mechanical and barrier properties are needed for an ideal edible flm for packaging. • Their primary functions as food packaging flm are to prevent mechanical, physical, chemical, and microbiological harm to the food. They preserve food consistency; prevent moisture loss; prevent bacteria development; and serve as a barrier to oxygen, water vapor, carbon dioxide, and volatile compounds.

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• For drug delivery systems, edible films act as a membrane or medium to deliver the drug to the targeted site or on a biological substrate. • Edible films and coatings for drug delivery systems should possess properties such as sufficient shelf life, good spreadability, sufficient tensile strength, and non-toxicity and be non-irritant. In recent years, nanotechnology has been a breakthrough for edible food packaging and drug delivery systems (X. He et  al., 2019). The incorporation of nanotechnology with antimicrobial nanoencapsulation has great advantages owing to its excellent protective system that is able to stand against diverse biological and environmental changes (Figure 3.4). This particular trend involves two steps of the process. First, antimicrobial compounds are packed into carriers, and second, their size is reduced to the nanoscale (10 −9 m) dimension. The types of nanofillers/ nanoparticles that are commonly incorporated into the biopolymers include nanofibers (e.g. starch, cellulose, chitin, and chitosan), nanoclay (e.g. MMT, kaolinite, hectorite, bentonite, saponite, organically modified clay), and metallic nanoparticles (e.g. silver, zinc oxide, titanium oxide, copper, copper oxide, gold, and silica) (Becerril et al., 2020). To truly comprehend the benefits of natural bionanocomposite systems, complete exfoliation and homogeneous dispersion of the nanoparticles in the biopolymer matrix are needed (Becerril et al., 2020). Various methods, such as solvent casting, in-situ polymerization, and melt manufacturing, have been widely used to create nanocomposite materials for a variety of uses, including antimicrobial packaging films (Bratovcic & Suljagic, 2019). These methods are further discussed in the next chapter.

FIGURE 3.4 changes.

Nanoencapsulation of antimicrobial agents, which protects food from diverse

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The technology of nanoencapsulation in the form of nanotubes, nanorods, nanocapsules, nanoshells, and nanospheres applied on bionanocomposite films and coatings has brought many benefits to food products, as shown in Figure 3.5, owing to the high surface area of the carriers relative to their bulk equivalents (Nile et al., 2020). Nanoemulsions, biopolymeric nanocarriers, solid lipid nanoparticles (SLNs), electrospun nanofibers, and nanoliposomes are some of the typical nano-encapsulating structures used in the development of novel nano-based active antimicrobial packaging systems with controlled release functionality (Nile et al., 2020). The nanocarrier systems are either starch-, protein-, or lipid-based. Although there are many advantages of carbohydrate and protein-based nanocapsules, they lack the ability to truly scale up due to the need for various complicated chemical or heat treatments that are difficult to monitor. Lipid-based nanocarriers, however, can be mass-produced industrially and have higher encapsulation performance and lower toxicity (X. He et al., 2019; Nile et al., 2020). In general, there are three models for bioactive integration into SLNs: the homogeneous matrix model, bioactive-enriched shell model, and bioactive-enriched heart model (Tan & McClements, 2021). The formulation components (e.g. lipid, lipophilic, or hydrophilic bioactive compounds and surfactant) and the fabrication approach determine the type of model obtained (e.g. hot or cold homogenization) (Nile et  al., 2020). When using the cold homogenization approach and integrating very lipophilic actives into SLNs using the hot homogenization procedure, a homogeneous matrix is primarily obtained. In this process, the bioactive compound is released by a dissolution process (Nile et  al., 2020). On the other hand, in the

FIGURE 3.5 The benefits of nanoencapsulation for bionanocomposite films that contribute high surface area of the carriers relative to their bulk equivalents.

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FIGURE 3.6

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Models for bioactive integration into SLNs.

cooling process from the liquid oil droplet, phase separation happens during which a bioactive-enriched shell can be attained, while a bioactive-enriched core is formed when a bioactive compound starts with precipitation, which result in lesser components being encapsulated (Figure 3.6) (X. He et al., 2019; Tan & McClements, 2021). Recently, Rizal et  al. (2021) discovered that seaweed incorporated with lignin nanoparticles significantly enhanced moisture resistance, tensile strength, Young’s modulus, elongation at breaks, and contact angle properties owing to high compatibility between the lignin and seaweed and strong interfacial interaction between the nanofiller and the matrix, as shown in Figure 3.7, where there is formation of a hydrogen bond between the hydroxyl groups of the nanoparticles and the matrix. The authors concluded that bioplastic films demonstrated significant functional properties, such as mechanical, thermal, and water barriers, that could be a successful choice to replace traditional petroleum-derived plastics in packaging material for a variety of applications. Biopolymers of various types are used to modulate the properties of films. Biopolymers in high demand include gluten, polymerized pullulan, sodium alginate, pectin, gelatin, and dextrin (Fahmy et  al., 2020; Shankar & Rhim, 2018; Sarfraz et  al., 2021). They have the potential to enhance solubility and increase stability. Furthermore, certain biopolymers, such as pullulans, have high tensile strength and temperature tolerance. Bionanocomposite films and coatings are commonly used in

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FIGURE 3.7 Incorporation of lignin nanoparticles in seaweed macroalgae. Reproduced from Rizal et al. (2021).

tissue engineering, pharmaceuticals, glass coating, food processing, wood coating, steel coating, medicine coating, and fruit coating, among other uses (Wani, 2021). However, depending on the demands of the final outcome, each film and coating application serves a variety of purposes. Another type of bionanocomposite film is mainly applied to agriculture field: mulch film. Mulch film is defined by some researchers as follows (Ayu et al., 2020; Surya et al., 2021): • Mulch film • It is the covering layer on soil used by farmers in crop production systems to preserve and insulate vulnerable plant root systems from extreme weather conditions. • It acts as a buffer to increase the nutrient profile of the soil. • Mulch films also helps to prevent erosion, improve the soil’s ability to retain moisture, reduce weed growth, and increase crop yield and precocity. Plastic mulch comes in 1,000–4,000-foot-long rolls that are 4–6 feet high and 1.0– 1.5 mil thick (Y. Zhang et al., 2020). It comes in a wide range of shades, from transparent to opaque. Colored mulches have recently been studied for their effect on pest control and plant yields. Reflective or silver mulches, for example, have been found to suppress onion thrip infestations (Cárcamo et al., 2021). Mulch films are usually applied once the fields have been leveled and smoothed, fertilizer has been added, and there is adequate soil moisture (Y. Zhang et  al., 2020). Since the soil is warmed by heat conduction, good uniform soil contact is important when using

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FIGURE 3.8 A contrast of mulch film and no mulch film on the soil surface. Reproduced from Surya et al. (2021).

black mulch. A mechanical mulch covering is the easiest way to spread mulch film evenly. Applying by hand is an alternative, but covering more than a half-acre can be tedious and time consuming (Ayu et al., 2020). As previously stated, numerous experimental trials have shown the effectiveness of plastic mulching, and measurements have been made to illustrate the difference with or without mulch (Surya et al., 2021), as can be seen in Figure 3.8. Mulch films, especially biodegradable mulch films, have played an important role in the sector of agriculture. The primary functions of these mulch films are as follows (Surya et al., 2021; Y. Zhang et al., 2020; Ayu et al., 2020): • Enhanced soil structure: Mulch films prevent soil from clumping together. Moisture and heat are easily trapped in soil. This would help to retain water in the soil. It also prevents the depletion of plant nutrients, which is very much suitable for drip irrigation. Furthermore, the plastic film deters people and pets from entering the field, enhancing the soil structure even more. • Soil protection: Farmers use mulch for a variety of purposes, one of which is to help the soil maintain heat as the winter months approach. This is because the majority of plants are temperature dependent, and vegetables, in particular, cannot tolerate the cold in the winter. Plastic mulch films, for example, warm the soil by up to 5°F. During the colder months, plastic mulching evenly maintains soil temperature, insulating temperature-sensitive plants. • Weed control: Plastic mulch films are able to suppress weed growth and do so over a wide area. Plastic mulch, as used in the greenhouse, prevents weeds from receiving the sunshine they require for photosynthesis. Weeds fail, as they are starved of sunshine, which saves the time and effort of manually removing weeds. • Fruit yield and growth: Plastic mulching helps to start growing crops earlier in the season. Mulching is important for higher-quality fruits and vegetables, as it serves as a barrier, preventing the fruits from coming into

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contact with the soil. This will prevent the transmission of diseases and rot. Furthermore, since the fruits are not in close contact with the soil, they mature even more cleanly. • Reduce root damage: Since the soil is not disturbed, plant roots may expand and disperse deeply and effectively into the ground. The most common form of mulch flm is polyethylene flm (Y. Zhang et al., 2020). Due to their non-biodegradability, these flms pose a danger to the setting. As a result, researchers all over the world have been looking for ways to replace polyethylene flm with biodegradable mulch flms derived from biological sources. However, not all biodegradable mulch flms decompose and become fertilizer (Surya et  al., 2021). According to R. Li et al. (2020), some biodegradable mulch flms have raised concerns due to their unintended consequences, where the remnants of these flms break down into microplastics. These microplastic particles are believed to pose environmental and organismal hazards. Due to increasing environmental concerns, researchers have developed biodegradable mulch flms that can decompose quickly in the soil. This type of mulch flm is typically made of biopolymer materials, specifcally polysaccharide-based, protein-based, and polylactic acid (PLA)-based materials (Hayes et  al., 2012; Cárcamo et al., 2021). To achieve the effciency of commercial mulch flms, nanotechnology has been used, in which nanofllers or nanoparticles are inserted into the polymer matrix with the main function to improve the physical and mechanical properties of the polymer matrix (Surya et  al., 2021). Several studies have been conducted, which include thermoplastic starch (Yin et al., 2020), seaweed incorporated with calcium carbonate (Abdul Khalil et al., 2018), and PLA bionanocomposites of lignocellulosic nanoparticles derived from yerba mate residues (Arrieta et al., 2018).

3.3.2

BIONANOCOMPOSITE HYDROGELS

A hydrogel is a water-swollen, crosslinked polymeric network made by combining one or more monomers in a simple reaction to form a three-dimensional (3D) network of hydrophilic crosslinked polymer chains that can also be found as a colloidal gel in which water serves as the dispersion medium (Ullah et al., 2015; Parhi, 2017). Another meaning is a polymeric substance that can swell and hold a large amount of water within its structure but may not dissolve in water. Because of their high water content, they have a degree of versatility that is somewhat similar to natural tissue (Guo et al., 2020). The inclusion of hydrophilic groups such as -NH2, -COOH, -OH, -CONH2, -CONH-, and -SO3H attached to the polymeric backbone gives hydrogels their ability to absorb water, while crosslinks between network chains give them resistance to dissolution (Catoira et al., 2019). Hydrogels can be cationic, anionic, or acidic in accordance with their ionic charges on the bound groups. Covalent crosslinks, ionic forces, hydrogen bonds, affnity interactions, hydrophobic interactions, polymer crystallites, and physical entanglements of individual polymer chains or a mixture of these interactions hold the hydrophilic polymer chains together as waterswollen gels (Tang et al., 2019).

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There are numerous types of hydrogels, which can be classified based on various factors, including their source, polymeric structure, physical and chemical composition, method of crosslinking, network electrical charges, and properties, as summarized in Figure 3.9 (Ahmed, 2015). In hydrogels, the presence of water is vital for the overall permeation of active ingredients in and out of the gel. Water in hydrogels can be divided into four major categories: bound water (primary and secondary), semibound water, interstitial water, and free water or bulk water, as illustrated in Figure 3.10 (Gun’ko et al., 2017).

FIGURE 3.9 Various types of hydrogels categorized under different bases. Source, polymeric structure, physical and chemical composition, technique of crosslinking, network electrical charges, and characteristics are the various factors to classify hydrogels.

FIGURE 3.10 Types of water associated with hydrogels.

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As polar hydrophilic groups come into contact with water, they are the frst to be hydrated, resulting in the formation of “primary bound water”. As the network expands, the hydrophobic groups will be exposed. These groups will then interact with water molecules to form hydrophobically binding vapor, also known as “secondary bound water” (Gun’ko et  al., 2017). When primary and secondary bound water are combined, it is referred to as “total bound water”. This water is an essential component of the hydrogel system and can only be isolated from it in exceptional circumstances (Gun’ko et al., 2017). Due to the osmotic pushing force of the network chains towards endless dilution, the network can consume more water. The covalent or physical crosslinks oppose this additional swelling, resulting in an elastic network retraction force. As a result, the hydrogel will hit a point of equilibrium swelling (Ahsan et al., 2021). The excess consumed water is referred to as “free water” or “bulk water”, which is thought to fll the spaces between the network chains, as well as the centers of larger pores, macropores, or voids (Gun’ko et al., 2017). There is a water layer called “semi bound water” that exists between the bound water on the surface of the polymeric monomer and the free water. Interstitial water exists in the interstices of a hydrated polymeric network that is physically bound but not connected to a hydrogel network (Ahsan et al., 2021). Nonetheless, if the network chain or crosslinks are degradable, the next focus is disintegration and/or breakdown, depending on the structure and composition of the hydrogel (Hamedi et al., 2018). Biodegradable hydrogels with labile bonds are thus advantageous in tissue engineering, wound healing, and drug delivery applications (Ahmed, 2015). These bonds can be found in the backbone of the polymer or in the crosslinks used to make the hydrogel. Under physiological conditions, labile bonds can be dissolved either enzymatically or chemically, with hydrolysis being the most common method. Apart from that, controlling the polarity, surface properties, mechanical properties, and swelling behavior of the hydrogel will affect its chemistry (Hamedi et al., 2018). Several studies have been done on incorporating nanofllers/nanoparticles into the biopolymer matrix with the goal to enhance hydrogel properties. These include bionanocomposite hydrogels based on sodium alginate (SA) as a polymer matrix and graphene oxide (GO) nanosheets with zinc as a crosslinking agent (Sabater i Serra et al., 2020). Through interactions with low concentrations of zinc, robust and strongly entangled networks were obtained from GO nanosheets scattered in SA matrices. The combined properties of these nanocomposite hydrogels make them appealing biomaterials in the feld of regenerative medicine and wound treatment (Sabater i Serra et  al., 2020). Hydrogels are also well known for the treatment of wastewater. Chemical crosslinking of cellulose, carboxymethyl cellulose (CMC), and intercalated clay in a NaOH/urea aqueous solution exhibited superabsorbent hydrogels. These hydrogels had superabsorbent properties in purifed water, better mechanical strength, and a high ability to remove microbes from wastewater than hydrogels made with only a polymer matrix (Qian, 2018). Hydrogels are much more well known in the pharmaceutical. According to Parhi (2017), Mohite and Adhav (2017), and Hamedi et al. (2018), hydrogels have unique properties that make them ideal for pharmaceutical, medicinal, and biochemical uses. Among these features are:

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• Since hydrogels are structurally and mechanically identical to the native extracellular matrix, they can serve as a supporting material for cells during tissue regeneration. • Their hydrophilic and crosslinked properties contribute to their excellent biocompatibility. • They are soft in feature, which encourages which encourages water uptake and hence the formation of hydrated but solid materials, similar to cells in the body. • Low interfacial stress between the hydrogel surface and body fuid decreases protein adsorption and cell adhesion, lowering the likelihood of a negative immune response. • The mucoadhesive and bioadhesive properties of certain polymers used in hydrogel formulations such as polyacrylic acid (PAA), polyethylene glycol (PEG), and polyvinyl alcohol (PVA) improve drug residence time on the skin/plasma membrane, resulting in increased tissue permeability. While hydrogels have numerous advantages, such as those mentioned before, they also have a number of disadvantages, one of which is the diffculty in forming them into predetermined geometries. Another disadvantage of hydrogels is that their reaction time is relative to their size, which is caused by sluggish water molecule diffusion (Mohite & Adhav, 2017).

3.3.3 BIONANOCOMPOSITE AEROGELS The term “aerogel” refers to a gel in which the liquid portion has been replaced by a gas (typically by the use of a supercritical drying technique) without affecting the overall structure, which also known as “open porous forms”. The end result is a solid with an exceptionally low density and a number of unique properties, the most notable of which is its usefulness as a thermal insulator and its extremely low density (Ganesan et  al., 2018). Due to its transparency and the way light scatters in it, it is also known as frozen smoke, solid smoke, or blue smoke. High porosity (usually greater than 80%), high thermal resistance (0.005–0.1 W.mK−1), low density (0.003–0.5g/cm 3), low dielectric constant (κ value ranging from 1.0 to 2.0), low refractive index (≈1.05), and high specifc surface area (500–1200 m 2/g) are among the critical properties that can be identifed in an aerogel (Guastaferro et al., 2021). Aerogels can be categorized into various types, with classifcation based on their visual characteristics, preparation techniques, unique microstructures, and chemical compositions. In past research, inorganic aerogels have been widely fabricated with the addition of nanofllers such as SiO2, Al2O3, and TiO2 in combination with synthetic polymers such as polyamide, polyurethane, and polyimide serving as the fundamental matrix for aerogel synthesis (Shi et al., 2021). However, concerns arise due to their negative environmental impact. Hence, more and more natural biopolymers, especially chitosan, cellulose, agar, agarose, pectin, and other polysaccharides and proteins, have been used to fabricate aerogels (Shi et al., 2021; Ganesan et al., 2018). Similar to hydrogels, the types of aerogels can be classifed on various bases such

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as appearance, preparation, chemical structures, and microstructures (Thapliyal & Singh, 2014). Aerogels can be manufactured in a variety of sizes and shapes in the forms of monolith, powder, and film since the initial liquid mixture can be adjusted during the gel forming process (Thapliyal & Singh, 2014; Ganesan et al., 2018). Figure 3.11 shows the common pathway of fabricating biopolymer-based aerogels. The method starts with dissolving biopolymers in water or chemical solvents to make organic aerogels (i.e. polysaccharides are generally soluble in aqueous solutions). The polymer chains then rearrange themselves into an open porous network as a result of solution gelation. Chemical, enzymatic, or physical crosslinking may cause gelation (Guastaferro et al., 2021). Due to the existence of functional groups localized on the backbones of several biopolymers, Van der Waals forces or hydrogen bonding may form (Shi et al., 2021). The porous structure of polysaccharidebased aerogels is determined by the drying technique. After drying, three types of solid materials can be produced: xerogel, cryogel, and aerogel. If a rigid material is dried under atmospheric pressure and at room temperature for several days, it is referred to as a xerogel (Abdul Khalil et al., 2020). The resulting materials are termed cryogels, as water (ice) within the hydrogel is sublimated by freeze-drying (Ganesan et al., 2018; Guastaferro et al., 2021). These samples, on the other hand,

FIGURE 3.11 The pathway of fabricating biopolymer-based aerogels. Reproduced from Abdul Khalil et al. (2020).

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have a macroporous structure with large, irregular pores. Supercritical CO2 drying is commonly used to synthesize aerogels (Abdul Khalil et  al., 2020; Guastaferro et al., 2021). This technique, when correctly executed by the operative pressure and temperature, is able to prevent the nanostructure from collapsing. Furthermore, it preserves the gel’s excellent textural properties (Zhang & Zhao, 2020; Guastaferro et al., 2021). The recent polysaccharide-based aerogels synthesized from natural ingredients show that their properties were on a par with or even better than those synthesized by synthetic polymer and silica. Among the signifcant properties possessed by natural biopolymer-based aerogels are high specifc surface area, low thermal conductivity, biodegradability, biocompatibility, and sustainability (Shi et al., 2021). Due to their unique characteristics, biopolymer-based aerogels have shown great potential in a wide range of applications. They can serve as effective thermal insulation, versatile drug delivery systems, valuable components for tissue engineering and regenerative medicine, effcient catalysts, sensitive sensors, reliable adsorbents, and essential raw materials for the production of carbon aerogels (Shi et al., 2021; Guastaferro et al., 2021). Leveraging their exceptional thermal, chemical, and functional properties, biopolymer-based aerogels fnd utility in separation processes and catalysis. They also serve as effcient drug delivery carriers due to their high specifc surface area, which leads to rapid drug release upon contact with liquid media. Furthermore, both unmodifed and drug-loaded biopolymer-based aerogels have been suggested for use as superabsorbents and for applications in wound healing due to their substantial pore volume and impressive swelling capabilities (Ganesan et al., 2018).

3.4 FABRICATION PROCESS OF BIOPOLYMER-BASED NANOCOMPOSITES The functionality of various biopolymers can be consolidated by conjugating them with nanoparticles, and the resulting bionanocomposites show characteristics of both biopolymers and nanoparticles. The wide range of bionanocomposite architectures and properties has broadened their applications. This section covers an overview of techniques for synthesizing bionanocomposites in both in academic research and industrial settings. The techniques to fabricate bionanocomposites are crucial, as they affect the properties of bionanocomposites and allow bionanocomposites to reach their full potential if the nanoparticles/nanofllers are distributed homogeneously (Bah et al., 2020; Unalan et al., 2014). One of the main issues in the manufacturing of polymer nanocomposites is the uniform and homogeneous dispersion of nanoparticles in the polymer matrix (Wei et  al., 2010). Nanofllers have a propensity to assemble and shape micron-sized fller clusters, limiting nanoparticle dispersion into the polymer matrix and degrading the properties of nanocomposites (Dantas de Oliveira and Augusto Gonçalves Beatrice, 2019; Varghese et al., 2021). Therefore, many attempts have been made by researchers to spread nanofllers evenly and homogeneously in the polymer matrix using chemical reactions, polymerization reactions, and fller content surface modifcations (Tanahashi, 2010). In the context of fabrication, the

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synthesis of bionanocomposites can be carried out following bottom-up strategies, which can be classified into a direct mixing approach, melt mixing/blending approach, in situ polymerization approach, and sol-gel approach. The basic structure of the bionanocomposites obtained are intercalated (and/or flocculated) or exfoliated (or delaminated) microstructures, which depend on the approach and materials used (Fawaz & Mittal, 2014; Bhattacharya, 2016; Jafarbeglou et al., 2016), as shown in Figure 3.12. The intercalation process will alter the structure of the host materials by expanding the interlayer spacing, weakening the van der Waals impact, and expanding the interlayer spacing, which inserts particles/ fillers between the layers, while the exfoliation process breaks the van der Waals force between the adjacent layers by exerting external force, causing the separation

FIGURE 3.12 The basic structure of bionanocomposites obtained are (a) intercalated (and/ or flocculated) or (b) exfoliated (or delaminated) microstructures.

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of layers with individual layers dispersed within the polymer matrix (Backes et al., 2020; Bhattacharya, 2016). Researchers have unveiled a multitude of manufacturing methods designed to overcome the limitations associated with biopolymers. Recently, they have strived to develop innovative manufacturing technologies suitable for both commercial and academic applications with the aim to streamline the synthesis of bionanocomposite materials, offering numerous benefits such as environmental friendliness, time efficiency, ease of operation, long-term sustainability, cost-effectiveness, exceptional physical and mechanical properties, and reliance on sustainable and eco-friendly resources (Bertolino et al., 2020; Ghorbani et al., 2021; Tuan Zainazor et al., 2020).

3.4.1

diRecT mixing appRoach

For preparation of bionanocomposites based on insoluble biopolymers, the direct mixing process mixes a soluble biopolymer and the dispersed nanofillers or nanoparticles in a solvent (Tanahashi, 2010), as shown in Figure 3.13. Note that the biopolymer and nanoparticles are dissolved or dispersed separately in solvent (Müller et al., 2017). These two solutions can be mixed together to allow intercalation to take place whereby the biopolymer chains replace the solvent molecules within the interlayer spaces of the nanoparticles (Li & Zhong, 2011; Zhan et al., 2017). Mixing by agitating the solvent can be done through magnetic stirring, shear mixing, reflux, or sonication. It usually results in thickening and gel-like precipitation (Salehiyan & Ray, 2018; Rafiee & Shahzadi, 2019). Upon solvent evaporation, the intercalated or exfoliated structure remains, which gives bionanocomposites. The properties of the bionanocomposites are determined by the interaction between the constituents of the matrix and the filler (Awan et al., 2021). Direct mixing is often utilized in biopolymers with nanoclay or nanosilicates for layer–layer assembly to form bionanocomposite films, coatings, or membranes (Salehiyan & Ray, 2018) (Awan et al., 2021). It starts with nanoparticles being dispersed in the soluble biopolymer mixture (Rafiee & Shahzadi, 2019). The interaction between the biopolymer solvent and the nanoparticles can cause a formation of stack

FIGURE 3.13 Direct mixing process for bionanocomposite fabrication.

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or layer-by-layer structure due to the van der Waals forces between the biopolymer solvent and the nanoparticles (Tanahashi, 2010; Müller et al., 2017). This approach is easy to execute and handle. Thus, many studies have used this approach to fabricate bionanocomposites, particularly in films (Leng et  al., 2019; Li & Zhong, 2011; Awan et al., 2021). However, the disadvantage of this approach is the poor compatibility of the matrix and the nanofiller, which leads to heterogeneous bionanocomposites when the nanofillers are not distributed evenly (Rafiee & Shahzadi, 2019). Apart from that, nanoparticles have the tendency to agglomerate upon solvent evaporation (Salehiyan & Ray, 2018). Another limitation of this approach is when it comes to fabricating bulk material such as hydrogels, as it needs a large amount of solvent to do so, which is cost ineffective (B. Li & Zhong, 2011). This approach is more appropriate to use to prepare bionanocomposites, especially films based on water-soluble biopolymers such as PVOH and polysaccharide-based biopolymers, as these types of biopolymers consist of polar sites to interact with silicate surfaces and exfoliate in the water, forming colloidal particles (He et al., 2017; Na et al., 2020).

3.4.2

melT Blending appRoach

The melt blending approach applies heat to the matrix at a suitable temperature and transfers it in a molten state to mix with the fillers (Müller et al., 2017). Nanoparticles can be combined directly with molten polymer rather than using solvent as medium, as shown in Figure 3.14. Many experiments have shown that polar interactions between polymers and the clay surface are important in achieving dispersion of particles (Tanahashi, 2010; Müller et al., 2017; R. Zhang & Zhao, 2020). The degree of nanoparticle dispersion or the compatibility between the matrix and the nanoparticles is influenced by two main factors, the processing conditions and the enthalpic interaction, where optimization of processing temperature and pressure is required. This method is very much affected by the sheer force applied, which needs to be monitored from time to time. This is because too much shear force can easily split the nanofibers into shorter fragments, hence decreasing the reinforcement effect and properties when high–aspect ratio nanoparticles are preferred (Jamróz et  al., 2020). Melt blending provides a cost-effective path and is environmentally friendlier due to the absence of solvent in the process. It also has the ability to process via extrusion and injection molding, which can be applied in industry. This approach has the advantage for mass production (Müller et al., 2017). The only limitation for this approach is the degradation of biopolymers that leads to polymer chain cleavage

FIGURE 3.14 Melt blending approach for bionanocomposite fabrication.

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due to mechanical shearing force or the temperature added during the process (Ke et al., 2012). Biopolymers such as soy protein, wheat gluten, zein, gelatin, PLA, and PHA are among the materials that are suitable to be processed in the extruder (Mangaraj et al., 2019). It is also an adaptable approach for thermoplastic biopolymers (Müller et al., 2017). As for this approach, layered clay minerals are often used as the filler because they give good intercalation of stacked clay mineral sheets (Dlamini et al., 2019). This approach is convenient and easy to carry out in a lab or even at industrial scale. This approach has also been used to fabricate biopolymer/cellulose nanofiber (CNF) bionanocomposites (Safdari et al., 2017; Dufresne, 2018).

3.4.3

In SItu polymeRiZaTion

In situ polymerization begins with the processes to distribute inorganic nanoparticles to the monomer or monomer solution, followed by monomer polymerization, which usually traps the exfoliated particles in the resultant polymer matrix. In this approach, the nanoparticle is distributed in a liquid monomer or a monomer solution. This process takes place when the filler/particulates swell in the liquid monomer solution (Geng et al., 2018). The swelling is caused by the process where the low molecular weight monomer percolates through the interlayers (Habibi et  al., 2014; Geng et al., 2018). Polymerization can be accomplished by the use of heat or radiation, the absorption of a suitable initiator, or the use of an artificial initiator or catalyst, allowing polymer formation to occur between the intercalated or exfoliated sheets (Rane et al., 2018), as shown in Figure 3.15. Utilizing economical materials, the simplicity of automation, and the flexibility to integrate with diverse heating and curing techniques, this method possesses the capacity to attain the optimal polymer chain length and molecular weight with the enhancement of both particulate and matrix distribution systems. Notably, it outperforms direct blending and melt blending in achieving superior exfoliation (Arzac et  al., 2014). However, some disadvantages of this preparation approach include minimal supply of available materials, a short time to complete the polymerization process, and the need for costly machinery (Mishra et al., 2014; Arzac et al., 2014). This approach is suitable for biopolymers and thermoplastic-based nanocomposites (Qin et al., 2020). Direct incorporation of well-dispersed nanoparticles in bulk polymer composites is possible with in situ polymerization. This approach is widely used in tissue engineering and food packaging applications (Doberenz et al., 2020; Fahmy et al., 2020).

FIGURE 3.15

In situ polymerization process of the fabrication of bionanocomposites.

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sol-gel appRoach

The sol-gel technique, also known as the template synthesis approach, is a bottomup technique that operates on the inverse theory of all previous approaches (Vaseghi & Nematollahzadeh, 2020). It is also known as the wet chemical procedure, which follows a sequential order, as displayed in Figure 3.16. Sol is a colloidal suspension of solid nanoparticles in a monomer solution, and gel is the three-dimensional interconnecting network generated between the phases during the gelation process. Solid nanoparticles are scattered in the monomer solution in this process, resulting in a colloidal suspension of solid nanoparticles (sol). By polymerization followed by hydrolysis, they form an interconnected network between phases (gel) (Baig et al., 2021). As seen in Figure 3.17, the polymer nanoparticle 3D network expands across the liquid. The polymer acts as a nucleating agent, promoting the formation of layered crystals. As the crystals expand, the polymer seeps through the layers. The solvent can be removed by evaporation or by critical drying, depending on the desired type of nanocomposite. Solvent removed through evaporation usually results in xerogel formation, while solvent removed by critical drying usually results in the formation of an aerogel. Research on the sol-gel approach is still lacking, and better comprehension of fundamental inorganic polymerization chemistry is required to expand the functionalities and potential applications of this approach (Abdul Khalil et  al., 2020). As of now, the sol-gel approach can be applied with a small investment, though the disadvantages need to be considered (Donato et al., 2017). With all the approaches mentioned, the major complication of processing bionanocomposites is the agglomeration of nanoparticles, which is accompanied by a lack of dispersion in the target formulation. Agglomeration and aggregation are caused by differences in surface area and volume effect (Zare et al., 2017). Hence, it is crucial to determine the dispersion efficiency and quality by characterizing them via different established techniques (Unalan et al., 2014), including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy (IR), and atomic force microscopy (AFM). Direct melt-compounding of the polymer with the fillers is the conventional practical approach for dispersing inorganic nanofillers in polymer matrices. The surface activity of the nanofillers, on the other hand, is exceedingly high. As a result,

FIGURE 3.16 The generic flow of the sol-gel approach, also known as wet chemical procedure.

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FIGURE 3.17 Sol-gel approach where the 3D polymer nanoparticle acts as a nucleating agent promoting the formation of layered crystals for the formation of xerogel and aerogel.

the particles appear to cluster closely. This is one of the most difficult issues in the production of filler/polymer nanocomposites. In light of this problem, a number of attempts have been made to distribute nanofillers uniformly in polymer matrices using methods involving organic modification of the surface or interlayer of nanofillers as well as a combination of sol-gel and/or polymerization reactions (Varghese et al., 2021). These developments, however, necessitate complicated chemical reactions, rendering them undesirable for industrial-scale manufacture of nanocomposites

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with a broad volume fraction spectrum of nanofllers and different combinations of fller and polymer content. In terms of industrial-scale fabrication of high-performance particle/polymer nanocomposites, a simple method for dispersing inorganic nanoparticles into various polymers through direct melt-blending, which does not require complicated reactions, is much preferred. Apart from that, the melt blending approach has shown greater advantages in terms of cost, practicality, and eco-friendliness compared to the other approaches. Another factor to note is the optimization of conditions to achieve better dispersion and exfoliation in bionanocomposites (Zare et al., 2017), for instance, the condition of temperature, where degradation can be avoided by running at lower temperatures or using more thermal stable modifcations.

3.5 THE PROPERTIES OF BIONANOCOMPOSITES This section aims to cover the most generic properties that are used to evaluate properties such as physical, mechanical, and thermal properties and biodegradability. According to Dantas de Oliveira and Augusto Gonçalves Beatrice (2019), bionanocomposites have some advantages in terms of their properties, among them being lighter than conventional composites due to high degrees of stiffness and strength achieved from much less high-density material, better barrier properties as compared to neat biopolymers, potentially superior mechanical and thermal properties, and biodegradability characteristics.

3.5.1

PHYSICAL PROPERTIES

Although there are many other physical properties that are evaluated for bionanocomposites, optical, water and gas barrier, and antimicrobial properties are common physical measurements used to determine the physical properties of bionanocomposite flms. Materials’ optical properties are especially important in some industries, where they can infuence either the effciency of the material or the consumer’s preference (Parola et  al., 2016). Both aspects should be taken into account when developing a new bionanocomposite. For instance, in the food industry, UV radiation (wavelengths less than 340 nm) should be avoided because it can induce photooxidation in photosensitive foods such as meat, alcohol, and milk, resulting in color, favor, and taste changes. On the other hand, high visible light transmittance (wavelengths between 340 and 800 nm) can be determined at the same time, as it helps users see through the packaging (visual inspection of processed food). The effect of bionanocomposite on the optical properties can be regulated in two ways (Unalan et al., 2014): • Using appropriate methods and procedures in the overall manufacturing phase • Selecting the most appropriate fller type In the exfoliation process of the fller, for example, physical, chemical, or mechanical approaches may be more or less effective depending on the fller. As for selecting

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the most appropriate filler type, it merely depends on the final products. If the final substance must be transparent, synthetic laponite (LAP) would offer better performance compared to one with sheet-like properties such as MMT, which is mainly due to the chemical composition and the well-defined dimensions of LAP compared to MMT (Unalan et al., 2014). In terms of barrier properties, bionanocomposites have excellent gas (e.g., O2 and CO2) and water vapor barrier properties (Dantas de Oliveira and Augusto Gonçalves Beatrice, 2019). According to previous studies, the form of fillers, aspect ratio of fillers, and nanocomposite composition all play a role in reducing gas permeability (Cader Mhd Haniffa et al., 2016; Mousa et  al., 2016; Shankar & Rhim, 2018; Wani, 2021). Generally, bionanocomposites featuring completely exfoliated clay minerals characterized by high aspect ratios tend to exhibit the most robust gas barrier properties. The underlying principle of this barrier effect, as elucidated by the authors, revolves around the ability of nanoparticles or nanofillers to establish a convoluted pathway within the matrix’s gallery structure (Dantas de Oliveira and Augusto Gonçalves Beatrice, 2019; Abdul Khalil et al., 2019). This phenomenon lengthens the transportation distance of gas or water permeate through the matrix, which results in higher barrier properties (Figure 3.18). Apart from the optical and barrier properties, bionanocomposites incorporated with antimicrobial agents such as silver nanoparticles, chitosan, chitin, and nanoclay are effective in microbial growth inhibition, as antimicrobial carriers and antimicrobial packaging films, and for enhancing shelf life due to the high surface-to-volume ratio and improved surface reactivity of nano-sized antimicrobial agents (Sharma et al., 2020; Shankar & Rhim, 2018). They are able to prevent more microorganisms than their larger-scale equivalents. Examples of bionanocomposites that have significantly exhibited antimicrobial properties were reviewed by Sharma et al. (2020). One example recorded that as silver nanoparticles and organoclay were integrated into gelatin, they demonstrated strong antimicrobial activity against food-borne pathogens such as E. coli and L. monocytogenes, with inhibition zones of 12 and 13 mm, respectively. Moreover, previous research has shown that oxidized starch/CuO

FIGURE 3.18 (a) Water vapor and gas passing through a clear path; b) water vapor and gas passing through a complex pathway.

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bio-nanocomposite hydrogels can serve as an antibacterial and stimuli-responsive agent with possible colon-specifc naproxen distribution in bionanocomposite hydrogels (Namazi et al., 2020).

3.5.2 MECHANICAL PROPERTIES Mechanical properties are determined by evaluating the composite’s stiffness and resistance to load exertion. To ensure product integrity and durability, composites must be able to withstand and last under such loads for a particular application (Rafee & Shahzadi, 2019). There are three main factors that affect the mechanical properties of bionanocomposites often discussed in research works (Abdul Khalil et al., 2019; Jamróz et al., 2020; Yin et al., 2020; Awan et al., 2021; Surya et al., 2021). These three factors are fller content, size and aspect ratio of the fller, and well dispersion of fllers. The mechanical properties of bionanocomposites are frequently found to be highly dependent on the fller material. Appropriate content of fllers is important to ensure the biopolymers are incorporated with nanofllers to demonstrate a signifcant improvement in the mechanical properties (Abdul Khalil et al., 2019). As for the incorporation of nanofllers, fller content lower than 5 wt.% is usually applied to prevent overloading and lead to fller agglomeration. The main reason for agglomeration could be due to the Van der Waals forces between the fllers (Costa et al., 2017). The size and aspect ratio of the fllers play an important role in affecting the mechanical properties of bionanocomposites. According to previous research, the tensile strength of hydroxypropyl methyl cellulose (HPMC)-based matrices integrated with smaller size AgNPs (40 nm) was 44.5% higher than the control (i.e. only contain biopolymers), whereas that incorporated with larger-size AgNPs (100 nm) was 26.5% higher than the control (De Moura et al., 2012). Mechanical properties can be affected by the well dispersion of nanofllers/ nanoparticles. This is to ensure no agglomeration occur in the bionanocomposite, which can lead to rigidity and weaker mechanical properties. In a bionanocomposite integrated with nanoclay, well dispersion of intercalated or exfoliated nanoclay resulted in signifcant changes in the mechanical performances. Another instance was found in cellulose/iron oxide bionanocomposites, where the well dispersion of 1 wt.% of iron nanoparticles showed a smooth morphology, resulting in 9.42 ± 0.6% and 45.16 ± 0.4% higher tensile strength and Young’s modulus compared to the control (i.e. only cellulose) (Yadav, 2018). Well dispersion of fllers can be affected by the interfacial adhesion between the fller and the matrix. As articulated by Taib and Julkapli (2018), when two separate materials are blended, merged, or mixed, interfacial adhesion occurs. This combination can result in better material dispersion in the matrices. Typically, to improve interfacial adhesion, a mixture of materials with similar properties, such as hydrophilic fllers and hydrophilic matrices or hydrophobic and hydrophobic materials, should always be used, resulting in a close bond between the two materials. The high rigidity and aspect ratio of nanoclay, along with the strong affnity through interfacial interaction between the polymer matrix and dispersed nanoclay, can be credited to the improvement in mechanical properties of polymer nanocomposites.

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THERMAL PROPERTIES

Thermal properties are the properties of a substance that are related to its heat conductivity. In other words, these are the characteristics that a substance exhibits as heat is applied to it. The thermal properties of bionanocomposites must be determined in order to guarantee that bionanocomposites can withstand the heat they exposed to. Bionanocomposites usually exhibit better thermal stability compared to pure biopolymers. Previous studies have shown that thermal and dimensional stabilities of bionanocomposites were improved with the incorporation of layered silicate clays; these are critical properties for large-scale production of thermoforming flms at elevated temperatures without shrinkage after processing of food packaging materials. The dimensional stability of clay/bionanocomposites is improved due to the higher modulus of nanoclay and lower thermal expansion coeffcient than the polymer matrix. Through differential thermal analysis (DTA) analysis, alginate/copper oxide (CuO) bionanocomposites showed signifcant enhancement of thermal stability compared to pure alginate (Saravanakumar et  al., 2020). Thermal stability was also found to be enhanced in ginger nanofber (GNF)/starch bionanocomposites, which was mainly attributed to the well-ordered orientation of the GNF cellulose in the composite moiety. This alignment was caused by the chemical reaction with the starch molecules, which resulted in higher thermal stability than GNF alone (Jacob et al., 2018). PVA/2D halloysite nanotube (HNT) bionanocomposites exhibit enhanced thermal stability when compared to pristine PVA, as evident from the shift in the maximum degradation temperature to higher levels. This improvement is attributed to the effective role of HNTs as heat and mass transfer barriers. Additionally, the natural hollow tubular structures of HNTs function as entrapment zones for volatile particles, thus augmenting thermal stability by impeding mass transfer during the decomposition process (M. Mousa and Dong, 2020). In general, the incorporation of nanofllers in the biopolymer matrix was found to improve thermal stability because the dispersed nanofllers act as an insulator for heat transfer and a barrier for mass transfer to the volatile products produced during thermal decomposition, and nanofllers also protect the polymer from the action of oxygen, dramatically increasing thermal stability under oxidation.

3.5.4 BIODEGRADABLE PROPERTIES One of the most essential properties of bionanocomposites is biodegradability. This is because this property helps to reduce critical waste problems. Polymer biodegradation can occur by any of the following mechanisms, which can occur alone or in combination (Vaezi et al., 2020): • • • •

hydrolysis enzyme-catalyzed hydrolysis solubilization ionization/microbial degradation

In general, biodegradation of polymers happens in two stages: depolymerization and mineralization (Rao et  al., 2019). Bionanocomposite packaging materials are

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expected to degrade quickly in the atmosphere after being disposed of (Vaezi et al., 2020). In general, it is understood that the biodegradability of biopolymer flms is greatly enhanced after incorporation with nanoclays. This applies to polylactic acid, where the breakdown begins with hydrolysis of the ester bond, then breakup into oligomer fragments, solubilization of oligomer fragments, diffusion of soluble oligomers, and fnally mineralization into CO2 and H2O (Rao et al., 2019). Another study done by Vaezi et al. (2020) showed that starch-based nanocellulose (NCC) particles and MMT bionanocomposites showed better degradation compared to the control (i.e. pure starch). It was explained that this behavior was caused by the dual effect from NCC and MMT. The higher degradation rate was mainly attributed to the addition of a hydrophilic fller, the NCC. On the other hand, MMT can limit water diffusion by improving barrier properties, as a result delaying hydrolysis and improving degradation. The authors also suggested that all of the investigated bionanocomposites began the disintegration phase earlier and at a faster pace than pure CS flms, implying their potential advantages in industrial applications where short biodegradation times are needed. Nevertheless, in some cases, nanofllers/nanoparticles are able to delay degradation. This was applied to starch/calcium carbonate bionanocomposites by Swain et al. (2018) and nanoclay/nanofbers/wood plastic by Saieh et  al. (2019). These fndings indicate that the structure of the nanoparticles and the existence of certain surface-modifying chemicals, such as quaternary ammonium cations, may infuence the degree of biodegradation of bionanocomposites (Salehiyan & Ray, 2018; Saieh et al., 2019; Mangaraj et al., 2019). By fne-tuning the biodegradation rate, this property can be used to produce bionanocomposite materials with the desired biodegradability properties.

3.6

CONCLUSION

Bionanocomposites, hybrid nanostructured materials based on natural polymers, have garnered signifcant research interest across various felds and have found diverse applications ranging from regenerative medicine to food packaging. This surge in interest has led to a growing number of scientifc publications, although studies have primarily approached the investigation of these biohybrid materials independently. However, there is potential to integrate bionanocomposites into a new interdisciplinary feld at the intersection of materials science, life sciences, and nanotechnology. Two primary reasons have driven the utilization of biopolymers in synthesizing nanocomposites. First, incorporating these natural polymers enables the production of biodegradable materials, which are crucial for developing environmentally friendly alternatives that help combat plastic waste pollution. Second, biocompatibility is vital for applying these biohybrids in areas such as food packaging and tissue engineering for regenerative medicine. The future advancement of bionanocomposites lies in developing novel materials with enhanced properties and multi-functionality. This feld of research holds immense potential due to the abundant and diverse availability of natural biopolymers and their advantageous combination with inorganic nanosized solids. The chapter underscores the importance of understanding the biodegradation behavior of biopolymers, particularly

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those sourced from microbial origins. Additionally, it highlights the signifcance of bionanocomposites derived from hydrogels and aerogels, explores the fabrication processes involved, and discusses their physical and mechanical properties.

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Wani, S. D. (2021). A review: Emerging trends in bionanocomposites. International Journal of Pharmacy Research & Technology, 11(1), 1–8. Wei, L., Hu, N., & Zhang, Y. (2010). Synthesis of polymer-mesoporous silica nanocomposites. Materials. https://doi.org/10.3390/ma3074066 Yadav, M. (2018). Study on thermal and mechanical properties of cellulose/iron oxide bionanocomposites flm. Composites Communications, 10. https://doi.org/10.1016/j. coco.2018.04.010 Yin, P., Chen, C., Ma, H., Gan, H., Guo, B., & Li, P. (2020). Surface cross-linked thermoplastic starch with different UV wavelengths: Mechanical, wettability, hygroscopic and degradation properties. RSC Advances, 10(73). https://doi.org/10.1039/d0ra07549c Zabihi, O., Ahmadi, M., Nikafshar, S., Preyeswary, K. C., & Naebe, M. (2018). A technical review on epoxy-clay nanocomposites: Structure, properties, and their applications in fber reinforced composites. Composites Part B: Engineering. https://doi.org/10.1016/j. compositesb.2017.09.066 Zare, Y., Rhee, K. Y., & Hui, D. (2017). Infuences of nanoparticles aggregation/agglomeration on the interfacial/interphase and tensile properties of nanocomposites. Composites Part B: Engineering, 122. https://doi.org/10.1016/j.compositesb.2017.04.008 Zhan, C., Yu, G., Lu, Y., Wang, L., Wujcik, E., & Wei, S. (2017). Conductive polymer nanocomposites: A critical review of modern advanced devices. Journal of Materials Chemistry C. https://doi.org/10.1039/c6tc04269d Zhang, R., & Zhao, Y. (2020). Preparation and electrocatalysis application of pure metallic aerogel: A review. Catalysts. https://doi.org/10.3390/catal10121376 Zhang, Y., Feng, R., Nie, W., Wang, F., & Feng, S. (2020). Plastic flm mulch performed better in improving heat conditions and drip irrigated potato growth in Northwest China than in Eastern China. Water (Switzerland), 12(10). https://doi.org/10.3390/w12102906 Zubair, M., & Ullah, A. (2020). Recent advances in protein derived bionanocomposites for food packaging applications. Critical Reviews in Food Science and Nutrition. https:// doi.org/10.1080/10408398.2018.1534800

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Biopolymers in 3D Printing Technology

4.1 3D PRINTING TECHNOLOGIES Three-dimensional (3D) printing, also known as additive manufacturing (AM), is a growing technology that has had a revolutionary impact on product fabrication for applications in several areas like healthcare and medicine, aeronautics, space, automotive, food, art, textile and fashion, architecture, and construction and has drawn increasing attention worldwide. Recently, the global market is moving towards the fourth industrial revolution (IR 4.0), which includes eight categories of digital transformation for the manufacturing industry: autonomous robots, simulated and augmented reality, the Internet of Things, cloud computing, cybersecurity, horizontal and vertical system integration, additive manufacturing, and big data analytics. In 2015, the Global Agenda Council on the Future of Software and Society at the World Economic Forum (WEF) anticipated that by 2025, we would witness the debut of the frst 3D-printed car, 5% of consumer products being manufactured using 3D printers, and the pioneering transplantation of a 3D-printed liver. Three-dimensional printing was frst described by Charles Hull in 1986 (Wang et al., 2017). The generic 3D printing process must start with 3D computer-aided design (CAD) information, as shown in Figure 4.1. It is originally generated by a CAD program such as AutoDesk, AutoCAD, SolidWorks, or Creo Parametric. Three-dimensional printing is derived from a Standard Tessellation Language, or STereoLithography (STL), fle by converting the fle into a G-fle via slicer software present in the 3D printer. Then, the G-fle divides the 3D STL fle into a sequence of two-dimensional (2D) horizontal cross-sections, which allows the 3D object to be printed, starting at the base, in consecutive layers of the desired material, essentially constructing the model from a series of 2D layers derived from the original CAD fle (Azlin et al. 2022). Simply put, 3D printing is a process of joining materials from 3D computer model data layer by layer using flaments of various types and sizes to make objects. Three-dimensional printing is good at reducing product development times and costs, and most importantly, it can fabricate designs and features unmatched by other methods of manufacturing. Table 4.1 lists advantages over traditional types of manufacturing techniques (Attaran, 2017). There are many types of 3D printing techniques, but no matter the technology involved, all are additive and build the object layer by layer (Jeffri et  al. 2022). According to ASTM International Technical Committee F42, 3D printing techniques can be generally classifed into four processes, material extrusion, vat

DOI: 10.1201/9781003416043-4

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FIGURE 4.1 Stages in the 3D printing process start from the file generated by CAD until the desired product is obtained. Reproduced from Ilyas et al. (2022).

TABLE 4.1 Advantages over Traditional Types of Manufacturing Techniques Area of Application Rapid prototyping

Production of spare parts

Small volume manufacturing Customized unique items

Very complex work pieces Machine tool manufacturing

Rapid manufacturing Component manufacturing

On site and on-demand manufacturing of customized replacement parts

Rapid repair

Advantages Reduce time to market by accelerating prototyping. Reduce the cost involved in product development. Make companies more efficient and competitive at innovation. Reduce repair times. Reduce labor cost. Avoid costly warehousing. Small batches can be produced cost efficiently. Eliminate the investment in tooling. Enable mass customization at low cost. Quick production of exact and customized replacement parts on site. Eliminate penalty for redesign. Produce very complex work pieces at low cost. Reduce labor cost. Avoid costly warehousing. Enable mass customization at low cost. Direct manufacturing of finished components. Relatively inexpensive production of small numbers of parts. Enable mass customization at low cost. Improve quality. Shorten supply chain. Reduce the cost involved in development. Help eliminate excess parts. Eliminate storage and transportation costs. Save money by preventing downtimes. Reduce repair costs considerably. Shorten supply chain. Reduce need for large inventory. Allow product lifecycle leverage. Significant reduction in repair time. Opportunity to modify repaired components to the latest design.

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TABLE 4.2 Descriptions of Various 3D Printing Technologies Processes Material extrusion (fused flament fabrication, liquid deposition modelling) Direct energy deposition (DED) Powder bed fusion (selective laser sintering) Binder jetting Vat photopolymerization (stereolithography)

Photopolymerzation (material jetting) Sheet lamination (laminated object manufacturing, composite additive manufacturing)

Technology Fused deposition modelling (FDM) • A material is melted and extruded in layers, one upon the other (this technique normally used in 3D printers at home). • An electron beam melts a metal wire to form an object layer by layer. Selective laser sintering (SLS) • A bed of powder material is “sintered” (hardened) by a laser layer upon layer until a model is pulled out of it. • Powder is bonded by a binding material distributed by a movable inkjet unit layer by layer. Stereolithography (SLA) • A beam of concentrated ultraviolet light is focused on the surface of a vat flled with liquid photo-curable resin. The UV laser beam hardens slice by slice as the light hits the resin. When a projector beams the UV light through a mask onto the resin, it is called digital light processing (DLP). Polyjet process • A photopolymer liquid is precisely jetted out and then hardened with a UV light. The layers are stacked successively. Laminated object manufacturing (LOM) • Layers of adhesive-coated paper, plastic, or metal laminates are glued together and cut to shape with a knife or laser cutter.

photopolymerization, sheet lamination, and powder bed fusion. Table 4.2 shows a summary of various 3D printing techniques (Goh et al., 2019).

4.1.1 FUSED DEPOSITION MODELLING Fused deposition modelling (FDM) was frst developed in the 1980s and was commercialized by Scott Crump of Stratasys Inc., USA, in the early 1990s (Mohan et  al., 2017). An FDM machine (Figure 4.2) fabricates a 3D model by extruding thermoplastic flaments and depositing the semi-molten flaments onto the bed platform layer by layer and subsequently changing the print material, which enables more user control over device fabrication for experimental use. The materials used to build 3D models are moved down by two rollers to the nozzle tip of the extruder of a print head, where they are heated by temperature control units to a semi-molten state. The semi-molten materials are extruded out of the nozzle and solidifed in the desired areas when the print head traces the design of each defned cross-sectional layer horizontally. The stage is then lowered, and another layer is deposited in the same manner, and these steps are repeated to fabricate a 3D structure in a layer-by-layer manner. Usually the outline of the part is printed

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FIGURE 4.2 Material extrusion methods (a) fused filament fabrication and (b) liquid deposition modeling. Reproduced from Goh et al. (2019).

first, followed by the internal structures (2D planes) layer by layer. In FDM, complex geometric components of the 3D model are produced by converting a file containing the 3D model into 3D STL format using CAD software. Subsequently, the STL file is brought into computer-aided manufacturing (CAM) software, where it is transformed into a tangible representation of the 3D model. The CAM software then divides it into fine layers, each comprising tool paths that guide the

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3D printing machine in depositing a continuous feedstock flament onto a surface. This gradual layer-by-layer approach is used to construct the 3D component (Owolabi et al., 2016). The key beneft of FDM is that it is inexpensive, and technological advancements have made it easier for users to communicate with machines and optimize tool paths. Additionally, the most widely used materials on FDM devices, polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS), are both commercially available at low prices. While the benefts of FDM are well known, due to component anisotropy and sometimes greater porosity, the mechanical properties of FDM printed parts are usually inferior to injection-molded parts. The degree of interfacial adhesion between discrete layers, tool (extrusion or print head) routes, and the inherent properties of the polymer in-plane cause anisotropy in AM part mechanical properties. (Wu et al., 2020).

4.1.2 LIQUID DEPOSITION MODELING Liquid deposition modeling (LDM) is a 3D printing technique that uses fuid or paste as a feedstock. Materials are injected selectively using a syringe connected to a computerized numerical control unit in LDM. Thermoset resins are simpler to handle than thermoplastics using the LDM technique, since the material used in LDM is in liquid form at room temperature. However, in order to extrude smoothly from the nozzle, epoxy resin ink requires unique rheological and viscoelastic properties. It’s worth noting that most ink dispersions made with these materials have a shear-thinning behavior, with a rising shear rate as viscosity decreases (Goh et al., 2019).

4.1.3

STEREOLITHOGRAPHY

Stereolithography (SLA), also known as the vat photopolymerization technique, was the frst advanced manufacturing process that was well known for producing low-porosity printed parts (0–5%). The polymerization of liquid resin or monomer exposed to electromagnetic radiation such as UV laser or electron beam is the basis for this method. At room temperature, polymerization takes place point by point, line by line, and eventually layer by layer. The build platform is lowered to a depth equal to the layer/cure thickness below the liquid resin/monomer, and a concentrated laser beam is guided on the liquid surface to cure it. SLA has been used to build fber-reinforced polymer composites (FRPCs) with reinforcements in the form of discontinuous fbers, continuous fbers, and fber mats to date. A single layer/crosssection, per the CAD model, is completed by rastering the beam, and then the build platform is lowered by the layer thickness, and the process is repeated. Cure depth (25–500 m) and width must be managed with suffcient beam size and scan speed for effcient bonding (interlayer and interscan). Since 80% of polymerization occurs during the actual SLA process, parts are processed using heat or photo-curing to complete the curing and improve mechanical properties after the build process is completed (Balla et al., 2019; Goh et al., 2019). Figure 4.3 shows the vat photopolymerization technique.

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FIGURE 4.3 Vat photopolymerization process that creates 3D objects by selectively curing liquid resin through targeted light-activated polymerization. Reproduced from Goh et al. (2019).

4.1.4

laminaTed oBjecT manufacTuRing

Laminated object manufacturing (LOM) (Figure 4.4a) is a method for creating a component from a stack of fiber sheets that incorporates additive and subtractive techniques. Each layer is cut with a laser, and then all of the layers are fused together with adhesive, pressure, and heat to minimize void material. Sheet material, which can be industrial prepreg sheets or any fiber preform, is used as the feedstock for LOM (Goh et al., 2019). There are two methodologies to this technique, LOM and composite-based additive manufacturing method (CBAM) (Figure 4.4b).

4.1.5

composiTe-Based addiTive manufacTuRing

Composite-based additive manufacturing starts with an aqueous-based solution that is inkjet-deposited on each layer of fiber sheet. The fiber sheet is then coated in a

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FIGURE 4.4 Sheet lamination process for creating a component from a stack of fiber sheets that incorporates additive and subtractive techniques (a) LOM and (b) CBAM. Reproduced from Goh et al. (2019).

thermoplastic powder film that only adheres to the aqueous solution. This procedure is repeated for all of the part’s layers. The fiber sheets are stacked, compressed, and heated in the oven to fuse the matrix for consolidation after the excess powder is removed. Last, the component is sandblasted to remove any remaining fibers and expose the finished product. One of the benefits of the sheet lamination technique is its ability to manufacture high-strength pieces as opposed to traditional methods (Goh et al., 2019).

4.1.6

powdeR Bed fusion

Selective laser sintering (SLS) (Figure 4.5) is a well-known powder bed fusion method for polymers. In SLS, thermal energy is used to selectively fuse regions of a powder bed in powder bed fusion. A thin layer of loose powder is placed on a build platform with a spreader, typically in a controlled atmosphere build chamber. A high-power laser beam scanned (using an X–Y scanner) over the bed surface according to the CAD model cross-section is used to fuse this powder layer. The contact of the laser with the powder produces enough heat to melt the powder, resulting in a solid cross-section. Overhang frameworks can be supported by the unaffected loose powder. After spreading a fresh layer of powder on the build platform, the process is repeated for all cross-sections by raising and lowering the feed box and build platform by one layer/slice thickness (100 m), respectively. After all of the layers have been completed, the pieces are cooled in a controlled atmosphere chamber and loose powder is extracted (Balla et al., 2019).

4.1.7 BindeR jeTTing Binder jetting is a manufacturing process that employs a liquid bonding agent and a powder-based material. Three-dimensional artifacts are created with the help of a

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FIGURE 4.5 Powder bed fusion method, also known as selective laser sintering. Reproduced from Pannitz and Sehrt (2020).

print-head that selectively jets liquid agent according to the desired cross-section, gluing powder material together. Binder jetting can be used in tissue engineering to create advanced and complex scaffold structures because of this. Devices for binder jetting are low cost, and unused powder can be reused in subsequent processes (Goh et al., 2019). The binder jetting process is also quite similar to SLS. Layer thickness, powder size/shape/distribution, feed powder to layer thickness ratio, drop volume, binder saturation, binder viscosity, print head speed, number of printing passes/layer, spearing speed, drying temperature and time, and number of foundation layers are some of the significant process parameters in the binder jetting method (Balla et al., 2019).

4.2 3D PRINTING MATERIALS The feedstock in additive manufacturing or 3D printing technologies must be formed into powder, sheet, filament, wire, or liquid, depending on the state that is compatible with the process. Polymers, metals, ceramics, and composites can all be used as 3D printing materials, depending on the 3D printing technology that is used.

4.2.1

TheRmoplasTic polymeRs

Thermoplastic polymers commonly used in 3D printing include polyamide (PA) or nylon, polycarbonate (PC), ABS, polymethyl methacrylate (PMMA), poly (lactic acid), polyethylene (PE), and polypropylene (PP). Material extrusion and powder bed fusion are two methods for processing thermoplastic polymers. Both methods depend on thermal layer adhesion, but the mechanisms are different. Material extrusion is usually done with amorphous thermoplastics, although powder bed fusion is best done with semicrystalline polymers (Balla et al., 2019; Alghamdi et al., 2021).

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Amorphous thermoplastics are the preferred choice for material extrusion processes because of their unique melting characteristics. ABS and PLA polymers exhibit a wide range of softening temperatures, extending up to what is known as the glazing temperature. This glazing temperature is crucial, as it enables the transformation of these materials into a high-viscosity state, making them ideal for extrusion through nozzles with diameters ranging from 0.2 to 0.5 mm. Material extrusion processes produce supporting overhangs, which must be removed during post-processing. A more advanced solution involving a two-head device with support made of wax-based or PVA materials is used, and a lattice structure made of the same material but with a lighter nature and lower strength in relation to the part is used. The supports are melted or dissolved away during the post-processing stage. PLA model materials are supported by PVA, a water-soluble support material. There are normally voids between the deposited paths of extruded material. As a consequence, the mechanical properties may be weak, and anisotropy effects may be present (Bourell et al., 2017). Powder bed fusion melts and fuses mainly semicrystalline powder feedstock using an infrared (IR) laser (typically a 10-mm CO2 laser) or an IR or UV heat source (lamps). Polyamide 12 is the most widely used semicrystalline material for powder bed fusion (nylon). Polyamide 12 (nylon) has a melting point of about 35°C, which is higher than the crystallization temperature. The substance melted by the laser remains molten and in thermal equilibrium with the surrounding unmelted powder when the AM fabricator temperature is set between these peaks. After the build, recrystallization occurs uniformly, reducing residual stresses to a minimum. Because of the support offered by the surrounding powder cake, the powder bed fusion process does not involve overhang supports in the case of plastics. Multiple nested parts can be used in the construction. By tweaking the processing parameters, nearly fully dense, low-porosity artifacts can be developed. A post infltration is needed for liquid pressure-tight applications (Bourell et al., 2017). Table 4.3 shows the various types of materials used in additive manufacturing, organized by process category and including commercial materials (Bourell et al., 2017).

4.2.2

THERMOSETTING POLYMERS

Monomers, oligomers, photoinitiators, and a number of other additives such as inhibitors, dyes, antifoaming agents, antioxidants, toughening agents, and other additives that help fne-tune the photopolymer’s behavior and properties are common photopolymer materials used in AM (Gibson et  al., 2015). Combinations of UV photoinitiators and acrylate monomers were the frst photopolymers used in vat photopolymerization (Lovo et al., 2020). One of the limitations of acrylate resin is that ambient oxygen inhibits its polymerization reactions. Vinylethers were a form of monomer used in the early stages of resin growth. However, acrylate and vinylether resins shrank by 5 to 20%, causing residual stresses to accumulate as parts were built layer by layer. Another thermoset resin that was introduced in the early 1990s was epoxy resin. Although complicating resin formulation, epoxy resin can be used to resolve several issues with other resins and provide signifcant benefts to the vat

Materials

Vat Polymerization

Material Jetting

Power Bed Fusion

Binder Jetting

Sheet Lamination

Directed Energy Deposition

X X

X

X X

X X X X X X X X X X X X X

X X X X X X X X X X X X

X X X X X

X X X X

X X X X

Biopolymers and Biopolymer Blends

Amorphous thermoplastics Acryonitrile butadiene styrene (ABS) Polycarbonate PC/ABS blend Polylactic acid (PLA) Polyetherimide (PEI) Polystyrene Semicrystalline thermoplastics Polyamide (nylon) Polypropylene Polyetheretherkeytone (PEEK) Thermoplastic polyurethane (elastomer) Thermoset Acrylics Acrylates Epoxies Aluminum alloys Co-Cr alloys Gold Nickel alloys Silver Stainless steel Titanium Ti-6Al-4V Tool steel

Material Extrusion

170

TABLE 4.3 Current Commercial Materials Directly Processed by 3D Printing, by 3D Printing Process Category

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photopolymerization process. Epoxies are cationically polymerized photopolymers. As epoxy monomers are reacted, their rings open up, allowing other chemical bonds to form. Ring-opening is known to cause minimal volume shift since the number and types of chemical bonds are nearly identical before and after the reaction. As a result, epoxy-containing stereolithography (SLA) compound resins shrink less than acrylates and are less prone to warping and curling. Epoxies are present in almost all commercially available SLA resins in large quantities (Bourell et al., 2017) due to the favorable mechanical properties of the resulting resins. Suitable cationic photoinitiators, activated by UV radiation, include onium salts and metallocene salts. Mixtures of acrylates, epoxies, and other oligomer materials make up commercial AM resins. Acrylates react quickly, while epoxies give the solid strength and durability. To form their respective polymer networks, acrylates polymerize dramatically, while epoxides polymerize cationically. The two types of monomers do not react with each other, but when combined, they form an interpenetrating polymer network (IPN). IPNs are a type of polymer blend in which both polymers are in network form and are created by two simultaneous reactions rather than a simple mechanical mixing phase. During the curing process, the acrylates and epoxies interact physically. The acrylate reaction increases photospeed, thus lowering the energy requirements for the epoxy reaction. In addition, the presence of acrylate monomers can reduce humidity’s inhibitory effect on epoxy polymerization. The epoxy monomer, on the other hand, serves as a plasticizer during early acrylate monomer polymerization; the acrylate forms a network while the epoxy is still liquid. The chain propagation reaction is likely favored by this plasticizing effect, which increases molecular mobility. As a result, the acrylate polymerizes more widely in the presence of epoxy, resulting in higher molecular weights than in the neat acrylate monomer. Also, due to the viscosity increase induced by epoxy polymerization, the acrylate in the hybrid system has a lower sensitivity to oxygen than in the neat composition, which may result in reduced diffusion of atmospheric oxygen into the material (Bourell et al., 2017). (Meth)acrylate-based resins are compatible with various commercial 3D printers and custom 3D printers. These resins have been used in a variety of applications, including shape memory polymer 3D printing, a siloxane-based hybrid polymer network, highly stretchable photopolymers, and functional biomaterials. PEGDA, UDMA, triethylene glycol dimethacrylate (TEGDMA), bisphenol A-glycidyl methacrylate (Bis-GMA), trimethylolpropane triacrylate (TTA), and bisphenol A ethoxylate diacrylate (BisEDA) are the most common (meth)acrylate monomer/oligomers used in 3D printing (Bagheri & Jin, 2019).

4.2.3 METALS Powder bed fusion and directed energy deposition are the main powder-based AM processes that are commercially used to manufacture quality metal parts. A metal wire feed can also be used instead of a powder feed in direct energy deposition (DED). Binder jetting is also used to produce metal parts. Polymer matrix parts are made that need furnace de-binding and sintering and/or infltration with a lower melting point metal (e.g., brass) to obtain dense metal parts. The set of common

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commercially available alloys is limited to pure titanium, Ti6Al4V, 316L stainless steel, 17-4PH stainless steel and 18Ni300 maraging steel, AlSi10Mg, CoCrMo, and nickel-based superalloys Inconel 718 and Inconel 625. This range is continually expanding with new entrants to the materials supply market. Precious metals such as gold, silver, or platinum have been processed indirectly by 3D printing of lost wax models but are currently also being directly used in selective laser melting (SLM) (Carlotto et al., 2014; Khan & Dickens, 2012). Several factors contribute to this limited metal palette. When fusion is involved, the metals generally must be weldable and castable to be successfully processed in AM. The small, moving melt pool is signifcantly smaller than the dimensions of the fnal part (typically on the order of 102–104 times smaller). This local hot zone in direct contact with a large colder area leads to large thermal gradients, causing signifcant thermal residual stresses and non-equilibrium micro-structures. For powdered feedstock, particles should preferably be spherical with a certain size distribution, which is different for powder bed fusion (PBF) and DED. The latter tends to be less sensitive to the dimensional qualities of the feedstock. A wire is also a suitable precursor material for certain DED processes, creating a larger melt pool than powder-based DED, allowing a higher production rate (Bourell et al., 2017).

4.3 BIOPOLYMERS IN 3D PRINTING 4.3.1 POLY (LACTIC ACID) ABS, a petroleum-based plastic, and PLA, a biobased plastic, are examples of thermoplastic polymers often used in FDM 3D printing. Yet PLA is the preferred polymer in FDM 3D printing since PLA comes from renewable resources and is biodegradable (Prasong et  al., 2020). Also, ABS will cause environmental issues like the emission of volatile compounds such as styrene, and ABS releases unpleasant odors (Andrzejewski et al., 2020). Owing to global environmental issues, PLA is considered a possible alternative to replace petroleum-based polymers, making it an ideal choice for industrial applications. In term of appearance, PLA is preferable for its wide range of available colors, translucencies, and glossy feel, and it has a semisweet smell compared to ABS (Wijk & Wijk, 2015). Though the mechanical strength of PLA is lower than that of ABS, PLA is a simple linear molecular chain structure, making it possible to enhance the mechanical properties of modifed PLA (Liu et al., 2019). Also, FDM machines are restricted to amorphous polymers or those with low levels of crystallinity, as they exhibit a low degree of polymer shrinkage, which is crucial to the accuracy of components produced (Stoof & Pickering, 2018). Low thermal expansion is advantageous for mitigating the internal stresses that occur during cooling. This makes PLA a more favorable choice when compared to ABS, as PLA exhibits lower glass transition, melting, and printing temperatures than ABS. Consequently, PLA enables precise dimensional control during the printing process, resulting in printed products that closely match the original 3D model (Wijk & Wijk, 2015; Liu et al., 2019). This characteristic of PLA signifcantly reduces the likelihood of warping effects (Cardoso et al., 2020; Andrzejewski et al., 2020). During cooling, the materials stretch and slightly shrink until the printed

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TABLE 4.4 Comparison of ABS and PLA Properties for 3D Printing Polymer Type

ABS

PLA

Extrusion temperature (°C) Bed temperature (°C) Moisture

225–250 80–110 ABS with moisture will bubble and sputter when printed but easily dry

Heat

Less deformation due to heating Plastic styrene smell Less color brightness

190–240 20–55 PLA with moisture will bubble and sputter when printed. It will not easily dry; can react with water and depolymerize at high temperatures Product can deform because of heat

Smell Color Hardness Fumes Details

Lifetime Environment

Very sturdy and hard Hazardous fumes Higher layer height, less sharp printer corners, needs a heated printer bed for less warping Longer-lifetime products Non-biodegradable, made from oil

Corn-like sweet smell Bright, shiny colors and smooth appearance Less sturdy than ABS Non-hazardous fumes Higher max printer speed, lower layer height, sharper printed corners, less part warping – Biodegradable, made from sugar, corn, soybeans, or maize

product starts to stiffen at 110oC and 56oC for ABS and PLA, respectively, based on their glass transition temperature, and the remaining shrinking is resisted by the stiffness of the material. The stresses are internally stored instead of being relieved by the warmer material’s ability to fow; thus, bending and warping occur. In addition, more warping is likely to happen for ABS compared to PLA because of its high glass transition temperature. Thus, printed PLA is relatively warm, but ABS is cool. In other words, without a heated bed or closed chamber, the bottom layers of the printed product will get cool fast, with shrinking and stiffening occurring at the same time for the ABS printed product (Andrzejewski et al., 2020; Bates-Green & Howei, 2017). PLA is the most popular polymer used among home printers, hobbyists, and universities other than in the industrial section. Table 4.4 shows a comparison between PLA and ABS in 3D printing (Cale Rauch, 2018).

4.3.2 POLY (LACTIC ACID)/POLY (BUTYLENEADIPATE-COTEREPHTHALATE) POLYMER BLENDS IN 3D PRINTING Recently, growing interest in research has focused on the improvement of PLA mechanical properties by polymer blending for conventional techniques, but only a few studies have been done on 3D printed polymer blending of poly (lactic acid)/poly

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(butyleneadipate-co-terephthalate) (PLA/PBAT) (see Table 4.5). The performance of the FDM printable product is highly based on the printing parameters, such as bed temperature, nozzle temperature, print speed, and layer height, all of which can affect the properties and printability of the product printed by FDM (Diederichs et al., 2019). Prasong et al. (2020) and Andrzejewski et al. (2020) observed that the performance trends in mechanical properties for FDM 3D printing and conventional injection molding techniques using PLA/PBAT blends are similar, primarily hinging on the content of the PBAT phase. In this context, the addition of PBAT provides a practical means to enhance the brittleness characteristics of PLA. Prasong et al. (2020) recommended that the optimal PLA/PBAT blend for 3D printing falls within the range of 10 to 30 wt.% PBAT. However, in a study by Andrzejewski et al. (2020), they noted that a 70/30 wt.% PLA/PBAT blend demonstrated superior mechanical properties. This improvement was attributed to favorable printing orientation and printing speed, which exceeded those achieved through conventional molding techniques. Both Prasong et al. (2020) and Andrzejewski et al. (2020) reported that printing quality is also affected by the viscosity of the polymer during printing, which is based on the printing temperature. According to Rahim et al. (2019), temperature is important in controlling the viscosity of polymer melting to ensure good fowability of the flament material during printing and thus result in good surface quality and optimal structural strength. High temperature may improve the mechanical properties of the printed sample, but it also affected the dimensional accuracy. Andrzejewski et al. (2020) mentioned the nozzle temperatures used to print PLA/PBAT blends were at 230°C for 10 to 20 wt.% PBAT content, and the temperature increased to 270°C for 30 wt.% PBAT content in a PLA/PBAT blend. However, it is said that the bed temperature has an important effect on the consistency of the printing layers and fnal mechanical properties of the printed sample compared to the nozzle temperature. Meanwhile, Liu et al. (2019) focused on cooling speed during printing, which is a crucial factor for interface bonding. Too slow a cooling speed causes bad surface quality, forming ability, flaments, and deformation shape of the printed product, whereas if the cooling speed is too fast, it causes insuffcient time for the flament to solidify, poor diffusivity of PLA molecular chains on the interface, and bad interface bonding, resulting in poor mechanical properties. Thus, the printing parameters play an important role in performance of the printed product. Table 4.5 presents the mechanical properties of 3D-printed PLA/PBAT blends.

4.3.3 3D PRINTING OF NATIVE CELLULOSE-BASED MATERIALS Cellulose, a homogeneous polysaccharide consisting of linear β-(1→4)-glucan with intra- and intermolecular hydrogel networks (Figure 4.3), is the most abundant renewable and biodegradable biopolymer worldwide. Development of cellulose-based materials as a new type of feedstocks for 3D printing technologies would open a new window to explore cellulose materials. Currently, the most intensively studied native cellulose-based 3D printing material is cellulose nanofbril (CNF) hydrogel. CNF hydrogel-based inks are biocompatible and can mimic the microenvironment of the extracellular matrix (ECM), which has been recently considered in biomedical

PLA/PBAT Manufacturing Method Filament Nozzle Bed and Nozzle Printing Tensile Tensile Elongation Impact (%) Diameter Diameter Temperature Speed Strength Modulus (%) Strength (mm) (mm) (°C) (mm/s) (MPa) (GPa) (J/m) 70/30 70/30 70/30 80/20

Twin-screw extrusion, capillary rheometer Twin-screw extrusion Mixer Twin-screw, single-screw extrusion

References

1.75

0.4

45, 210

25

46.9

–

225.8

–

2 – 1.75

0.5 0.2 0.35

60, 270 60, 200 50, 210

10 70 40

48.8 22.9 44.8

2.289 – 2.513

37.3 – 34

327 (Andrzejewski et al., 2020) – (Lyu et al., 2020) 5.5 kJ/m2 (Wang et al., 2020)

Biopolymers in 3D Printing Technology

TABLE 4.5 Mechanical Properties of 3D-Printed PLA/PBAT Blends

(Prasong et al., 2020)

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applications (Liu et al., 2016). However, the high hydrophilicity and inherently entangled state of CNF limit the increase of ink concentration at a given viscosity and storage modulus. Efforts have been made to directly dissolve or utilize native cellulose in other forms. Native cellulose cannot be readily dissolved in common solvent systems due to the strong H-bond network and crystalline structure. Ionic liquids (ILs) are a new class of “green” cellulose solvents with low melting temperature. Dissolution of cellulose is achieved by disruption of the native cellulose H-bond network, with new H-bonds formed between cellulose and anions and hydrophobic interactions with cations. Anti-solvents, such as water, ethanol, and acetone, can be added to regenerate cellulose and recycle the ILs (Gupta & Jiang, 2015). Liu et al. (2019) dissolved different cellulose materials, including bacterial cellulose (BC), microcrystalline cellulose (MCC), and dissolving pulp, in an IL (EmimAc) with solid concentration up to 4% and applied it for direct ink writing (DIW) printing. Complex patterns of 2D structures and multilayered cylinder structures were printed and coagulated with water. However, the spatial resolution of the printed structure and recycling of ILs still need to be addressed. Cellulose nanocrystals (CNCs) prepared from acid hydrolysis may offer advantages over cellulose IL dissolution and CNF in terms of the increase of concentration at a given rheology requirement for 3D printing. Siqueira et  al. (2017) dispersed CNCs in water to prepare different concentrations of suspensions/gel inks (0.5–40%) for DIW and yielded cellulose-based structures with a high degree of CNC particle alignment along the printing direction, offering the opportunity to print cellulosic architectures with tailored mechanical properties. Native cellulose can also be utilized in 3D food printing by serving as dietary fber, bulk flling agents, rheological modifying ingredients, or reinforcing ingredients owing to its indigestibility, excellent mechanical performance, and high viscosity of native cellulose. Lille et al. (2018a) incorporated CNF gel in a 3D printing ink formula as a reinforcing ingredient for the development of healthy customized 3D printed foods. CNF was found to improve the shape stability of the printed structures and to decrease the hardness of the dried objects, revealing its potential for 3D printing healthy fber-rich and structured foods.

4.3.4

3D PRINTING OF CELLULOSE DERIVATIVE-BASED MATERIALS

The super-molecular structure of cellulose, that is, high crystallinity degree and rigid intra/intermolecular hydrogen bond networks, restricts its application. However, the abundant hydroxyl groups on the cellulose surface open opportunities for chemical modifcation or functionalization, such as esterifcation, etherifcation, selective oxidation, graft copolymerization, and intermolecular crosslinking reactions, bringing novel opportunities to exploit for various applications such as in food, cosmetics, medicine, and pharmacy (Liu et al., 2015). The interfacial compatibility problem between polar native cellulose and the nonpolar polymer matrix is the key limitation for the application of native cellulose, which has excellent mechanical performance as a reinforcing agent in composite manufacture. To improve compatibility with PLA, surface modifcation of cellulose

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was conducted using a titanate coupling agent via coordination exchange (neoalkoxy) or solvolysis (monoalkoxy) (Murphy et al., 2016). The modifed cellulose was found to improve dispersion and interfacial adhesion between the cellulose and the PLA, leading to higher mechanical strength of the composite flament than neat PLA alone. Porous scaffold prototypes were successfully printed with FDM using cellulose-reinforced PLA flaments, offering the opportunity for rapid production of fully degradable biocomposite 3D prototypes for applications in the biomedical, automotive, and construction sectors. Similarly, to improve the dispersibility of CNC and interfacial bonding after DIW and UV polymerization, Siqueira et  al. (2017) applied an acetylation reaction for CNC surface modifcation by using methacrylic anhydride. The modifed CNC was dispersed in a mixture of photopolymerizable monomer and photoinitiator. After DIW printing and curing, the yielded structures were found to have a high degree of CNC particle alignment along the printing direction, providing the opportunity to print cellulosic architectures with programmable reinforcement along prescribed directions. (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO)-mediated oxidation and carboxymethylation are two common approaches to prepare CNF by introducing negative charges on the cellulosic fber surface followed by mechanical processing (Liu et al., 2014). The yielded CNF hydrogels with shear thinning behavior and high zero shear viscosity have been tested as inks for 3D bioprinting. Rees et al. (2015) prepared CNF hydrogels with TEMPO-mediated oxidation and a combination of carboxymethylation and periodate oxidation and used them to print 3D porous structures for wound dressing application. Both CNF-based inks were found to inhibit bacterial growth, suggesting potential application as wound dressing materials. The CNF prepared from TEMPO-mediated oxidation failed to print constructs with acceptable resolution and desired tracks due to the limited low consistency (0.95%) but relatively high viscosity. The one prepared by carboxymethylation and periodate oxidation was suitable for use as a bioink in terms of high consistency (3.9%) and appropriate rheological properties. The printed 3D structures have fne tracks with open porosity and the potential to carry and release antimicrobial components for wound dressing. Carboxymethylated CNF with high surface charge and higher consistency (2%), which holds the 3D shape after deposition, has been investigated as ink to print 3D structures (Håkansson et al., 2016). By controlling the solidifcation process, such as air drying, air drying with surfactants, solvent exchange before drying, and freeze drying, 3D printed structures with the desired mechanical, surface texture, and porous structure can be obtained (Figure 4.4A). Incorporation of functional ingredients, such as conductive carbon nanotubes, offers additional functionality to the printed products. As shown in Figure 4.4B, C, conductive ink composed of carbon nanotubes and CNF can be printed in a CNF hydrogel matrix in different layers with fne resolution. After drying, two over-crossing conductive lines with decent electrical conductivity were separated by the insulation CNF layers, suggesting a potential for fabrication of sustainable commodities such as packaging, textiles, biomedical devices, and furniture with conductive parts. Pattinson and Hart (2017) demonstrated the manufacture of fully dense cellulosebased materials using cellulose acetate by 3D printing. Cellulose acetate has 80% of

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the surface hydroxyl groups replaced by acetate groups, which effectively disrupted the hydrogen bonding network of native cellulose and made it possible to dissolve in acetone with high consistency (25–35%). Solid cellulose acetate structures with isotropic strength and high toughness were built upon the evaporation of acetone. By incorporating antimicrobial agents, direct 3D printing of cellulose acetate-based objects with tailored biochemical functionality (e.g. antimicrobial properties) can be achieved, enabling customization of medical instruments in a short time (e.g. forceps with customized tips; Figure 4.4D). Methylcellulose was utilized by Schütz et  al. (2015) to enhance the temporary printability of a 3% alginate with limited viscosity. The addition of methylcellulose signifcantly enhanced the bioink viscosity, enabling accurate and easy 3D bioplotting of constructs with tailored architecture and high shape fdelity, after which the methylcellulose was released from the scaffolds during the following cultivation. Embedded mesenchymal stem cells in 3D printed methylcellulose-alginate scaffolds were found to maintain their differentiation potential with high viability. The temporary integration of methylcellulose into alginate-based bioink with low concentration allowed the generation of scaffolds with high shape fdelity and stability while keeping the advantages of low-concentrated alginate bioink for cell embedding. A research team from Aalto University working within the project “Design Driven Value Chains in the World of Cellulose” studied 3D FDM printing of thermoplastic cellulose derivatives (Ali, 2015). The printability of the pure cellulose derivative was improved through plasticization, which reduced melt viscosity, improved the layer adhesion, and lowered glass transition temperature, allowing manufacture of a variety of cellulose-based 3D structures. Similarly, within the same project, oxidized cellulose hydrogel, cellulose-based plastics, and pulp fber composites were studied for textile printing (Figure 4.4E, F) (Tenhunen et al., 2018). Structures with the desired look or feel, such as hard or soft, strong or brittle, stiff of fexible, or porous or dense texture, can be printed directly on substrates or fabrics or form self-standing structures and can fnd applications from medical science to personalized sportswear (Chaunier et al., 2018).

4.3.5 3D PRINTING OF CELLULOSE COMPOSITE-BASED MATERIALS Incorporation of other ingredients into cellulose-based ink formulations may signifcantly improve ink properties such as processability, printability, mechanics, and bioactivity (Guvendiren et al., 2016). CNF hydrogels have ideal structural similarity to ECM and excellent rheological properties for 3D printing. However, pure CNF printed structures lack high shape fdelity (fne line resolution), suffcient mechanical properties for handling during transplantation, and long-term structure stability, limiting their application in the 3D printing of complex scaffolds in different biomedical applications (Gatenholm et al., 2016). Alginate also possesses shear thinning behavior at high shear forces, but the low viscosity at a zero shear rate makes it diffcult to print 3D constructs. Inks composed of CNF and alginate keep the shear thinning behavior and high zero shear viscosity of CNF, while the alginate allows crosslinking by divalent ions to maintain long-term shape fdelity and structural integrity after 3D printing. Markstedt et  al. (2015) optimized and evaluated the printability and

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biocompatibility of this composite ink. Anatomically shaped cartilage structures, such as a human ear and sheep meniscus, were 3D printed with high shape fdelity and stability (Figure 4.5A–C). Human nasoseptal chondrocytes were incorporated into the CNF-alginate composite ink for 3D bioprinting, and the results suggested that the shear forces during mixing and the crosslinking process could result in a decrease of cell viability. Although cell viability can be recovered after seven days of culture, cells embedded in the structures (e.g. thickness exceeds 150–200 μm) may gradually lose viability due to the limited diffusion of oxygen/gas, nutrients, growth factors, and waste product (Lee Ventola, 2014). To overcome this issue, Martínez Ávila et al. (2016) printed a chondrocyte-laden patient-specifc auricular construct with open porosity for oxygen, nutrients, and waste diffusion (Figure 4.5D). The bioprinted constructs showed excellent shape and size stability after bioprinting, and long-term 3D culture and the CNF-alginate bioink was found to support redifferentiation of human chondrocytes, re-establishing and maintaining their chondrogenic phenotype, suggesting the usefulness of the bioink for auricular cartilage tissue engineering and many other biomedical applications. Similarly, Nguyen et  al. (2017) designed human-derived induced pluripotent stem cells (iPSCs) and irradiated human chondrocyte-laden CNF-alginate/hyaluronic acid composite inks to mimic cartilaginous tissue for potential cartilage lesion treatment. In the composite inks, the CNF was supposed to mimic the bulk collagen matrix, alginate stimulates proteoglycans, and hyaluronic acid serves to substitute for hyaluronic acid in native cartilage. Bioink of CNF-alginate was found to maintain the pluripotency of stem cells and support the new generation of cartilaginous tissue with collagen expression and high cell density, suggesting the bioprinting of iPSCs with CNF-alginate bioinks as a promising treatment to repair damaged cartilage. Alginate sulfate with the potential to support the chondrocyte phenotype has also been incorporated into CNF hydrogels to formulate bioinks for cartilage bioprinting (Müller et al., 2016). The composite bioink was found to promote bovine chondrocyte spreading, proliferation, and collagen II synthesis. However, the bioprinting process signifcantly compromised cell proliferation, especially in the case that used nozzles and valves with a small diameter and high extrusion pressure and stress. Currently, CNF-alginate composite ink formulation has been successfully commercialized with the brand name of CELLINK and has been evaluated in vitro and in vivo with human chondrocytes, human dermal fbroblasts and keratinocytes, neural cells, mesenchymal stem cells derived from bone marrow, and adipose tissue and iPSC cells derived from chondrocytes (Gatenholm et al., 2016). For example, Henriksson et al. (2017) bioprinted the living adipocyte-laden CELLINK bioink and a bioink composed of nanocellulose and hyaluronic acid (CELLINK-H) for 3D bioprinting in adipose tissue engineering. The adipocytes laden in 3D-printed structures, especially the CELLINK-H, produced better adipogenic differentiation and a more mature cell phenotype with larger lipid droplets than conventional 2D culture systems, suggesting a promising method for adipose tissue engineering. A clinical study was recently conducted by transplanting a bioprinted cell-laden CELLINK bioink scaffold in a subcutaneous pocket of mice (Möller et al., 2017) (Figure 4.5E). The histological, immunohistochemical, and mechanical analysis suggested that the 3D-bioprinted scaffolds have excellent structural integrity,

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shape fdelity, and good mechanical properties after 60 days of implantation, and the scaffolds can result in cartilage synthesis, suggesting a potential application of CELLINK bioinks in 3D bioprinting cartilage tissue for application in reconstructive surgery (Liu et al., 2019).

4.3.6 3D PRINTING OF HEMICELLULOSE-BASED MATERIALS Hemicelluloses, which mainly include xylans, glucomannans, arabinans, galactans, and glucans, are a type of heterogeneous polysaccharide and are widely distributed in biomass. Hemicelluloses have long been studied and widely used in different areas, such as food and feed, medicine and pharmaceutics, and papermaking due to their biocompatibility, biodegradability, nontoxicity, and specifc therapeutic activities (Liu et al., 2015). However, utilization of hemicelluloses either natively or their functionalized products as feedstocks for 3D printing has rarely been studied. Recently, a spruce wood hemicellulose (O-acetyl galactoglucomannan, GGM) was utilized to partially replace a synthetic PLA as feedstock in 3D FDM printing (Xu et al., 2018). The blends of hemicelluloses and PLA with a varied ratio up to 25% of GGM were evenly mixed with a solvent casting approach and were extruded into flaments by hot melt extrusion. 3D scaffold prototypes were successfully printed from the composite flaments by FDM 3D printing (Figure 4.3). As a pioneer exploration, this study demonstrated the feasibility of applying biorenewable hemicelluloses as a novel material candidate for FDM 3D printing, which has explored a new route to utilize hemicellulose-based biopolymers in 3D printing for versatile applications in, but not limited to, biomedical devices. By mimicking the hemicelluloses’ natural affnity to cellulose, Markstedt et al. (2017) mixed a tyramine-substituted xylan with the CNF to introduce a crosslinking property of the all-wood-based inks for 3D printing. The tyramine substitution degree of xylan-tyramine and the mixture ratio were found to infuence the printability and crosslinking density of the all-wood-based inks and the swelling properties of the printed structure, providing the opportunity to tune the mechanical and structural properties of the printed object, which might even transfer to 4D printing (Liu et al., 2019).

4.3.7 3D PRINTING OF STARCH-BASED MATERIALS Starch is another type of abundant polysaccharide produced by higher plants as energy storage and is composed of linear amylose and highly branched amylopectin with structures of α-(1→4) linked glucan and the -(1→4) linked glucan with α-(1→6) branch linkages, respectively (Fraser-Reid et  al., 2008). Utilization of starch or starch-based polymers, such as thermoplastic starch and PLA, as feedstocks in 3D printing technologies for a variety of applications has drawn increasing interest from both academic research and in industry applications owing to its renewability, biodegradability, biocompatibility, and high abundance in nature (Davachi & Kaffashi, 2015) A three-powder blend ink consisting of 50 wt.% corn starch, 30 wt.% dextran, and 20 wt.% gelatin and water-based binders was adapted for 3D printing of porous

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scaffolds for tissue engineering applications (Lam et  al., 2002). After 3D printing, post-processing by heat treatment (dried at 100°C for 1 h) was applied to sinter the three blend powdered particles together via necking connections, enhancing the strength of the scaffolds and also the water resistance. Although the fabricated porous scaffolds, which consisted of natural polymers and a water-based binder, showed potential in tissue engineering, further biocompatibility study is needed. In 3D food printing, starch is commonly utilized as a thickening/gelling agent or rheological modifer and also serves as an important carbohydrate source. Incorporation of starch into the ink formulation improves ink printability by offering the ink shear thinning behavior and also shape stability of the printed structure. For example, starch has been applied to facilitate the 3D printing of sodium caseinate (Schutyser et al., 2018), mashed potatoes (Liu et al., 2018), lemon juice gel (Yang et al., 2018), and protein- and fber-rich food materials (Lille et al., 2018b) by working as thickening/gelling agent. Similar to cellulose, the structure of starch also has abundant hydroxyl groups, offering numerical chemical modifcation/functionalization possibilities to produce starch derivatives, starch-based polymers, or plastics for 3D printing (BeMiller & Whistler, 2009). For instance, a thermoplastic starch (TPS)/acrylonitrile-butadienestyrene copolymer was prepared by using a compatibilizer (styrene maleic anhydride) to generate hydrogen bonds as well as strong van der Waals force between TPS and ABS and was subjected to flament extrusion and FDM 3D printing. The results revealed that the TPS/ABS flaments and printed 3D structures had superior mechanical properties and thermal resistance compared to that of commercial ABS, suggesting that the potential of using biomass polymeric materials and their derivatives with excellent 3D printing processibilities and physical performance would be highly promising in the future (Kuo et al., 2016). Through fermentation with micro-organisms and polymerization processes, PLA can be produced from starch in a sustainable and green approach. Owing to a variety of benefts, including biocompability, nontoxicity, biodegradability, ease of processing, low cost, excellent mechanics, and the green features of its synthesis routes from renewable resources, PLA makes up the majority of the FDM 3D printing feedstock and has drawn increasing interest for biomedical applications, such as scaffolds, implants, prostheses, drug-based delivery systems, and anatomical model fabrication (Saini et al., 2016; Tyler et al., 2016). Depending on the ratio of PLLA and PDLA, the mechanical performance and crystalline structure of the polymers can be tailored for specifc requirements of different biomedical applications. For example, sutures and orthopedic devices that need high mechanical strength and toughness can be fabricated with a high ratio of PLLA, while drug delivery systems that require a porous and monophasic matrix can be produced with amorphous PDLA (Shah Mohammadi et al., 2014). While the advantages of PLA make it a promising material for various applications in the biomedical and pharmaceutical felds, it’s essential to acknowledge its limitations. PLA’s hydrophobic nature, limited ultimate elongation strain, absence of cell motif sites, production of small particles, and release of acidic byproducts during degradation, which can trigger foreign-body infammatory reactions, are factors that must be considered. These drawbacks may potentially lead to clinical complications

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and impose restrictions on its utilization in the feld of biomedicine (Suganuma et  al., 1991). Therefore, increasing efforts have been made to enhance the hydrophilic properties, to increase the cell motif sites, to introduce bioactivity, to improve the ultimate elongation strain, and to address the formation of acidic biodegradation products (Manavitehrani et al., 2016; Ulery et al., 2011). For example, Wang et al. (2016) printed PLA-based scaffolds treated with a cold atmospheric plasma (CAP) modifcation for enhancing hydrophilicity and cell motif sites. The ends of the PLA chains (−CH3) on the surface were reacted with reactive oxygen species (peroxides, superoxides, hydroxyl radicals, and atomic oxygen) and converted to hydrophilic groups, such as −CH2OH, −CHO, and −COOH. Meanwhile, the CAP treatment raised the surface temperature of the PLA to ca. 40°C, which partially melted and reformed the polymer, allowing conversion from microscale to nanoscale surface features. Results showed that the enhanced surface hydrophilicity and nanoscale roughness by CAP surface modifcation signifcantly promoted both osteoblast (bone-forming cells) and mesenchymal stem cell attachment and proliferation, suggesting promising applications in bone tissue engineering (Wang et al., 2016). Along with physical treatment, surface functionalization with bioactive molecules is another approach for improving biomaterial bioactivity. For instance, Kao et al. (2015) functionalized 3D-printed PLA scaffolds via a mussel-inspired surface coating with polydopamine to accelerate protein adsorption and the cell cycle of human adipose–derived stem cells. Similarly, surface coating of PLA scaffolds by physical sorption and covalent bonding of collagen and polymers (polyethyleneoxide-polypropyleneoxide copolymers) or incorporation of bioactive glass has also been adapted to improve scaffold bioactivity and to tune cell response (Serra et al., 2013). The surface-coated or modifed PLA scaffolds are demonstrated to regulate cell organization, adhesion, and proliferation and to induce osteogenesis and angiogenesis differentiation, providing a very promising tool to regulate stem cell behavior, and they may serve as an effective stem cell delivery carrier for bone tissue engineering (Lin & Fu, 2016). Modifcation or functionalization of PLA to tune the mechanical performance of 3D printed PLA-based scaffolds also play a keys role in its biomedical application. Senatov et al. (2016) designed and fabricated a PLA-based porous scaffold with a shape memory effect by FDM 3D printing for potential application as a self-ftting implant in bone defect replacement. PLA, which serves as a bioresorbable matrix, and the bioactive fller hydroxyapatite (HA) powder (15 wt.%) were mixed in a screw extruder, and the PLA/HA composite flaments were extruded for FDM 3D printing. The 3D-printed PLA-based porous scaffolds were shown to have the shape memory effect after heat activation, which changed the polymer chain mobility. The scaffolds were found to be able to withstand up to three compression–heating–compression cycles without delamination. However, the higher activation temperature (70°C) for the shape memory effect compared to the body temperature is an obstacle for the application, and therefore further study to lower the activation temperature is needed (Senatov et  al., 2016). Similarly, Zhang et  al. (2016) prepared a PLA/HA (15 wt.%) composite scaffold with a pore size of 500 μm and 60% porosity by a mini-deposition system (FDM) (Jiang et al., 2012). Incorporation of HA can not only

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enhance the mechanical performance of the scaffolds (Jiang et al., 2012) but also offer scaffolds better biocompatibility, biodegradability, and osteoinductive activity than pure PLA scaffolds to enhance bone formation, showing potential application in bone tissue engineering as a promising candidate for bone defect repair (Liu et al., 2019; Zhang et al., 2016).

4.3.8 3D PRINTING OF ALGAE-BASED MATERIALS Algae-based materials, such as alginate, agarose, and carrageen, extracted from brown and red algae, represent another group of naturally derived biopolymers from marine biomass for 3D printing ink formulation, especially for 3D bioprinting (Axpe & Oyen, 2016). Alginate is one of the most widely explored seaweed anionic polysaccharides and is composed of β-(1→4)-linked D-mannuronic acid (M block) and α-L-guluronic acid (G block) units arranged with varying proportions of GG, MG, and GM blocks (Liu et  al., 2015). It has been recognized as the most commonly employed material as bioink formulation in 3D bioprinting for a variety of biomedical and pharmaceutical applications, such as wound healing, cartilage repair, bone regeneration, and drug delivery, owning to the benefts of nontoxicity, biocompatibility, biodegradability, non-antigenicity, and ease and mildness of gelation (Chimene et  al., 2016; Liberski, 2016). By mixing it with multivalent cations, such as Ca2+, alginate is ionically crosslinked to form a hydrogel, offering an ECM-mimicking environment for cell incorporation and survival, good printability, structural integrity, and mechanical stiffness. By adjusting or using alginate with different G/M ratios, molar mass, solid content, and cell density, the rheological, mechanical, and macro- and micro-structural properties of the alginate-based bioink can be tuned to ft the requirements of different bioprinting methodologies and biomedical applications (Grigore et al., 2014; Luo et al., 2015). However, some undesirable features, including poor long-term structural integrity and mechanical properties, slow and uncontrolled degradation kinetics, and the bioinert property (non–cell-adhesion by itself), should be carefully considered in the design and development of alginate-based bioink for bioprinting (Chung et al., 2013). For instance, the poor mechanical performance could be addressed by combining ionic and covalent crosslinking (Duan et  al., 2014; Kesti et  al., 2015; Rutz et  al., 2015); by the incorporation of other components such as hydroxyapatite (Bendtsen et  al., 2017), polycaprolactone (Daly et  al., 2016), β-tricalcium phosphate (Diogo et al., 2014), and nanocellulose (Möller et al., 2017); and by assisting deposition of a sacrifcial layer (e.g. PEG) that can be removed after printing (Armstrong et al., 2016). The slow degradation kinetics of native alginate can be addressed by oxidation with sodium peroxide or periodate (Jia et al., 2014), by controlling the alginate molar mass and distribution (Boontheekul et al., 2005), by enzyme-assisted degradation (e.g. alginate lyase) (Wong et al., 2000), or by incubation with sodium citrate that accelerates the dissolving of alginate hydrogel by chelating calcium ions from the CaCl2 crosslinked alginate hydrogel (Wu et al., 2016). The bioinert property of native alginate can be addressed by conjugation of cell attachment ligands (e.g. peptide sequence Arg-Gly-Asp, i.e. RGD) or with incorporation of other biomacromolecules, such as fbrin, collagens, gelatin, chitosan, hyaluronic acid, avidin protein,

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and various growth factors to offer the desired bioactivities (Leppiniemi et al., 2017). Agar and its purifed agarose are galactose-based polysaccharides derived from red algae and seaweed such as Gracilaria, and Gelidium agarose is a linear polysaccharide composed of alternating (1→4)-linked (3,6)-anhydro-α-L-galactose and (1→3)-linked β-D-galactose, while agar is a mixture of the predominant agarose and agaro-pectin composed of partially sulfated (1→3)-linked D-galactose (Serwer, 1983). These galactose-based polysaccharides, especially agarose, have proved suitable for bioprinting owning to their cytocompatibility, stable mechanical properties, and mild temperature-dependent gelling conditions with a starting gelling temperature of 37°C and full crosslinking at 32.7°C (Malda et al., 2013). Different cell lines have been encapsulated into the agar, agarose, and their composite hydrogels with other polymers such as alginate, collagen, matrigel, and chitosan as bioinks in bioprinting for potential applications in tissue engineering, regenerative medicine, and drug discovery (Navarro & Garcia, 2016; Wei et al., 2015).

4.3.9

3D PRINTING OF CHITOSAN-BASED MATERIALS

Chitosan, a cationic polysaccharide, is composed of β-(1→4)-linked nacetyl-Dglucosamine and deacetylated D-glucosamine units (Figure 4.6C). It is the deacetylated form of chitin that naturally exists in the exoskeletons of crustaceans (crab, shrimp shells, and insects), as well as the cell wall of fungi and yeast, and it is the second richest natural polymer in nature after cellulose (Dutta, 2016; Rinaudo, 2006). Chitosan-based biomaterials with unique advantages have drawn increasing attention for application in medical and pharmaceutical areas, such as tissue engineering, drug delivery, and wound healing, owing to their biocompatibility, bioresorbability, biodegradability, antimicrobial activity, mucoadhesivity, and low toxicity, as well as natural abundance (Elviri et  al., 2017). Generally, chitosan is soluble in acidic conditions, such as 2% acetic acid, and the amine groups in chitosan are protonated to confer poly-cationic behavior. Chitosan chains expand into a semi-rigid rod conformation due to ionic repulsion between the charged groups (NH3+), and thus chitosan ink exhibits shear thinning behavior, which is benefcial for the extrusionbased 3D printing (Wu et al., 2017). Chitosan or chitosan-based composites in the form of hydrogels or pastes, flaments, or strips have been applied in a variety of 3D printing technologies for potential bone tissue engineering, cartilage tissue regeneration, and drug screening. These printing approaches include FDM (Wu, 2016); stereolithography SLA (Morris et  al., 2017); and different nozzle extrusion-based 3D printing technologies such as double-nozzle assembling (Shengjie et  al., 2009), rapid prototyping robotic dispensing (Ang et al., 2002), direct printing (Almeida et al., 2014), and lowtemperature manufacturing (Elviri et  al., 2017). However, poor mechanical resistance and degradability in physiological conditions, as well as acidic dissolution, make it diffcult to incorporate living cells into chitosan-based inks in bioprinting for biomedical applications. Neutralization and gelation in alkaline conditions (Almeida et  al., 2014; Elviri et  al., 2017), derivatization, crosslinking, or combination with other polymers are required prior to cell seeding or implantation (Pandey et  al., 2017). For instance, hydroxyapatite, pectin, and genipin have been incorporated into

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chitosan for mechanical reinforcement of chiton-based inks or 3D-printed scaffolds in bone tissue engineering (Demirtaş et al., 2017). Combination with other natural or synthetic polymers, such as alginate, gelatin, fbrinogen, laminin, and thermoplastics, would potentially offer many opportunities to overcome these challenges and may allow the introduction of desired properties (Liu et al., 2019).

4.4 CONCLUSIONS The emergence of 3D printing as a groundbreaking technology holds the promise of revolutionizing traditional product fabrication. Nevertheless, a signifcant challenge that needs to be addressed before its widespread adoption in various sectors is the scarcity of high-quality printing materials that are also environmentally friendly. Among the abundant and renewable sources of biobased materials, such as lignocellulosic materials; seaweed materials; and exoskeleton materials from crustaceans like crabs, shrimp shells, and insects, naturally derived biopolymers offer great potential for diverse 3D printing technologies. By modifying or combining these biopolymers with other ingredients to enhance their processability and the functionality of printed products, we can expand their application areas. Future trends in 3D printing material development will likely involve utilizing nature-derived biopolymers, either in their original form or as functionalized products, as feedstocks for various 3D printing technologies. Recent advancements and proof-of-concept demonstrations have confrmed the technical feasibility of developing naturally derived biopolymers as feedstock formulations. However, there are still several challenges to overcome, including material processability; degradation; and chemical, biological, and mechanical properties, before fully realizing the potential of biopolymers. Once these hurdles are addressed, 3D printing technology is poised to greatly facilitate the widespread adoption of naturally derived biopolymers in various felds. Consequently, it is foreseeable that future material development in 3D printing will progressively shift towards the utilization of these environmentally friendly feedstocks derived from nature.

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Applications of Biopolymer Blends and Biopolymer-Based Nanocomposites

5.1 INTRODUCTION Throughout human history, biopolymers have served various purposes, such as food, clothing, and materials for furniture. However, starting in the 1950s, petroleum-based polymers emerged as the primary source for the development and production of commercial products, such as plastics, which are now ubiquitous. In light of growing environmental concerns and limited fossil fuel resources, researchers have shifted their focus towards fnding alternatives to petroleum-based products. The turning point came with the signing of the environmental treaty on December 11, 1997, which became law on February 16, 2005. This treaty addressed pressing environmental issues, including greenhouse gas emissions, climate change, and the adverse impacts on ecosystems stemming from toxic waste, pollution, resource depletion, and environmental degradation (Attaran et al., 2017; Abdul Khalil et al., 2023). The substitution of biobased polymers for synthetic plastic products has emerged as a signifcant trend across numerous industrial sectors. This shift is regarded as a viable alternative to synthetic polymers, given that biobased polymers are derived from renewable resources. They offer a positive environmental impact and can naturally replenish through the Earth’s cycles. The renewable nature of biopolymers leads to the development of new sustainable products driven from renewable feedstock with many advantages, including disintergrability and degradability, improved possibilities for recycling, a lack of toxic components, and high biocompatibility (Luzi et al., 2019). Biopolymers can be from a living organism or materials that need to be polymerized but come from renewable resources. Biopolymers that come from living organisms include polysaccharides (starch, cellulose), protein (wheat gluten, soy protein, gelatin), and polyester-like poly-alkoxy alkanoates (PHAs) which are produced from bacteria. The latter includes polylactic acid (PLA); polycaprolactone; polyhydroxyalkanoates; polyethene glycol; and aliphatic polyesters such as PBS, PVA, and polyurethanes (PUs) (Attaran et al., 2017; Chassenieux et al., 2013; Tang et al., 2012). Despite the positives claimed regarding biopolymers, they also suffer from drawbacks that may limit their application, such as high production cost and limited mechanical and thermomechanical properties concerning synthetic polymers (Kumar et  al., 2017; Luzi et  al., 2019). Consequently, developing DOI: 10.1201/9781003416043-5

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eco-friendly polymeric blends can overcome these limitations (Luzi et al., 2019). During the last three decades, a new method for polymeric material preparation by blending two or more materials for various uses has been observed. Blending biopolymers with other biopolymers is a promising technique for preparing polymers with “tailor-made” properties (functional properties and biodegradability). The combination of biopolymers with biodegradable synthetic polymers is also considered a trend, as it provides a way to reduce the overall cost of the materials and offer a method of modifying both properties and degradation rates (Asyraf et al., 2023). Biopolymer-based nanocomposites are advanced materials formed from a fascinating interdisciplinary area that consists of biology, materials science, and nanotechnology. The superiority of these new materials comes from the various choices of biopolymers and reinforcements available, including clays, nanocellulose, metal nanoparticles, and graphene. The interaction of nano-scale fllers acts as a bridge in the polymer matrix and leads to the enhancement of the mechanical properties of the nanocomposites. This combination of these materials also adds a new dimension based on biocompatibility and biodegradability. Therefore, bio-nanocomposites are of great interest for biomedical technologies, including tissue engineering, medical implants, dental applications, and controlled drug delivery (Chassenieux et al., 2013). Moreover, the utilization of biopolymer-based nanocomposites extends across a range of sectors, including construction, automotive, aerospace, biomedical, cosmetics, and packaging. These industries have embraced and extensively explored the application of nanotechnology as a viable strategy to enhance the specifc properties necessary for their respective domains. The application of nano-scale fllers as reinforcing materials helps turn biopolymers into biocomposites with superior mechanical strength and other characteristics essential for specifc applications (Luzi et al., 2019; Pacheco-Torgal, 2016).

5.2 BUILDING AND CONSTRUCTION Biopolymers fnd extensive application in the realm of building and construction. In certain scenarios, biopolymers present unique benefts when contrasted with their synthetic counterparts. Notably, biopolymers are gaining wider environmental acceptance, particularly within the context of interior home construction.

5.2.1

SUPERPLASTICIZERS

Chemical admixtures are essential and important for manufacturing modern concrete and have been a focus of development in high-tech felds. Superplasticizers are widely used to produce fowable, strong, and durable Portland cement in the presence of plasticizers. The addition of a superplasticizer helps enhance properties, especially after the material hardens (Xun et al., 2020). The utilization of biopolymers in concrete has a long history, with instances of natural occurrences. Historically, the exploration of biopolymer applications dates back to the construction techniques pioneered by the Romans. During their fourishing era, the Romans made noteworthy advancements, including the development of a cementitious substance known as opus

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caementitium, which served as the fundamental building material for iconic structures like the Roman Colosseum. Marcus Vitruvius Pollio (84–10 BC) described the invention of construction and materials in his famous encyclopedia, De acrhitectura decem, which explained the role of admixtures in improving building materials, for example, the use of air lime mortars with the addition of vegetable fat (Giavarini et al., 2006). The Romans were celebrated for their innovative techniques aimed at enhancing building materials. They ingeniously incorporated dried blood as an airentraining agent, while biopolymers, such as proteins, played a role as set retarders for gypsum. Similarly, the Chinese employed substances like egg white, fsh oil, and blood in the construction of the Great Wall due to their reinforcing properties (F. Pacheco-Torgal, 2016; Plank, 2005). In 1507, mortars based on lime mixed with small amounts of vegetable oil added during the slaking process were applied in the construction of the Portuguese fortress Nossa Senhora da Conceicao, located on Gerum island, Ormuz, Persian Gulf (Pacheco-Torgal, 2014; Pacheco Torgal & Jalali, 2011). Later in the 20th century, lignosulfonates, a biopolymer for concrete plastifcation, were introduced. This marked a pivotal moment in the use of biopolymers, specifcally lignosulfonates, for plasticizing ordinary portland cement (OPC) concrete. This was the frst functional polymer in construction to be used on a large scale (Plank, 2005). OPC concrete, a typical civil engineering construction material, is the most-used material on earth. To date, around 15% of total OPC concrete production contains chemical admixtures to modify the properties. Example of biopolymers used in concrete include lignosulfonate, starch, chitosan, pine root, extract, protein hydrolysate, and vegetable oils. The addition of polymers in Portland cement offers improved mechanical properties. The function of polymer addition is actually to reduce permeability, diminish the number of large pores, refne them, and hinder the propagation of cracks. Also, the addition of polymer helps to better organize the microstructure of concrete. The fnal products will become more elastic, tough, and resilient. Similarly, this mechanism also represents the function of biopolymers. Concrete with higher toughness and resilience is required for structures designed for relevant thermal variations or dynamic loads followed by fatigue (Bezerra, 2016). Also, biotech admixtures received increasing attention due to their high biosynthesis rate compared to plant-based biopolymers. These admixtures include sodium gluconate, xanthan gum, curdlan, and gellan gum (F. Pacheco-Torgal, 2016). Table 5.1 shows a list of biopolymers and synthetic polymers used for chemical admixtures. Although OPC and dry-mix mortars use mostly biopolymers, a great diversity of biomixtures for over 500 different products is currently being used by building material industries and is expected to grow with the expansion of urban land. The greatest application of biopolymers in the construction industry relies mostly on inorganic binders and coatings of building materials that mostly focus on the feld of rheology control (dispersing/thinning or thickening) and water retention. Biopolymer admixtures also serve many other purposes, includes dispersing/thinning effects, viscosity enhancement, water retention, set acceleration and retardation, air-entrainment, de-foaming, hydrophobing and adhesion, and flm-forming.

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TABLE 5.1 Major Milestones in Chemical Admixture Technology for Construction Year of Introduction 1920s 1940s 1960s 1962 1970s 1980s 1980s 1990s

Admixture Chemistry Lignosulfonate Lignite Xanthan gum Melamine, naphthalene condensaters Cellulose ethers Vinyl-sulfonate copolymers Polycarboxylate copolymers Polyaspartic acid

Function

Type of Admixture

Concrete plasticizer Bentonite thinner Viscosifer Concrete superplasticizer

Biopolymers Biopolymer Biopolymer Synthetic polymer

Water-retention agent Water-retention agent Concrete superplasticizer Biodegradable dispersant, retarder

Biopolymer Biopolymer Synthetic polymer Biopolymer

According to Bezerra et al. (2016), these biopolymers are usually utilized in these forms: • Powder form: biopolymers that can be either added to cement or diluted water for concrete preparation. Example: chitin, chitosan, and so on. • Liquid form: biopolymers that are usually diluted in water for concrete preparation. Example: latex materials (rubber, avelos, Araucaria, dilutan, welan, xanthan, gelan, gutta-percha, guar), and so on. • Fiber form: biopolymers that have undergone the biopolymerization process that will increase the tensile strength of the concrete. The function of biopolymers as plasticizers continues to be explored by researchers to enhance the properties of concrete. For example, a biopolymer-based admixture to enhance the viscosity properties of concrete was studied by Leon-Martinez et al. (2014) using nopal mucilage and marine brown algae extract. In this study, the authors reported the success of nopal mucilage and marine brown algae to increase the viscosity of concrete. The study discovered concrete prepared with 0.25% of nopal mucilage produced excessive bleeding and segregations, whereas concrete with 0.25% of marine brown algae extract seemed to be the best admixture, presenting good spread, low segregation, and bleeding with high air content (